Books? I don't know mate, I have a feeling that majority of us here didn't read any books as far as sports betting goes. We just follow our favorite team and maybe go to the sports bookies and just learn what those odds are like ML (Money Line), or handicap betting like the -/+.

It was all about the knowledge of the games itself and the teams that are playing. Maybe he didn't follow the sports that much that's why he is losing big money. I will just talk about experience and says that I have started to follow the sports that I bet since I was a little like basketball and boxing. And as time, teams and players, but the basic of the games still remains the same.

And then I just bet on sports bookies and try to learn those odd lines. I might have lose some money in the beginning, but that is the most important lessons that I have learn in the years that I gamble on sports.

Aside the TOS of casino sites, we can practically just begin to gamble and make good memories, discover new skills and good profits out of it but that is strictly based on lucky base games.

Just saying that... Without reading books we can still play on some sport bets just with the basic knowledges like... The football which you just have to understand that your roles to play their is to decide what outcome the game would be, what team you are betting on and which one you are against which then involves analytical or technical thoughts.

Though in playing strange games, it could be necessary to follow guides to how the games can be played so it does not feel like we do not know what we are doing.
I really had not read books towards gambling guides if not as we discusses here in the forum so I don't know what book to recommend but I think that of your friend can as well go through the game events sites maybe there could be illustrated of its guides.

But then, it is not mean he is going to be a professional player despites readings.

A book that will make him to begin to win? Tell him not to deceives himself. That gambling and betting are not about making money but about entertaining oneself. The best that you can tell him is to let him know about using just 1% of his income on gambling, that if he increases the percentage, it should not be more than 5%. I did not read any book for gambling.

I usually don't like people to play gamble blindly so it is very important for new gamblers to know how gambling works, this are some books that will help him know about sport betting how it works and how to become a winner.

1. The Everything Guide to Sports Betting By Josh Appelbaum
2. The Basics of Winning Sports Betting
By Avery Cardoza
3. The Logic of Sports Betting By Ed Miller & Matthew Davidow.
4. The Odds: One Season, Three Gamblers And The Death Of Their Las Vegas
by Chad Millman
5. Gambler: Secrets from a Life at Risk
by Billy Walters

One of the things you will always see the author's of this books talk about is that you as a gambler you need to watch the sports you always like to gamble on, for example if you always gamble on soccer then you most try to watch soccer know things about soccer, teams that are inform and that are not, also try to read about past history of some teams, there are some teams no matter how not informed they are when they are about to play a particular team they become inform and will play very well for example Manchester United and Chelsea when ever they play it is always a difficult game and the team that's not inform that season becomes inform on that match.
Also try listening to your mind if you listen to it and do as it says and it works for you then always listen to it but if it those not work then you forget about it and focus on studying a game before betting.

I don't think books will help when it comes to sports betting or maybe it's just me. IMO, it's about watching most of the games that you can and that's when you will realize how it all works. I have been a fan of the NBA since I was a kid and I didn't read any books about it. Mostly just tapes and CDs of my favorite games is what I have like the Finals of Michael Jordan and the Chicago Bulls and then keeping myself updated with recent movements and changes.
Right now, what helps me the most in analyzing the game is all on the internet and it does work many times where I win most of my bets. I think the love for the sport is what will help him acquire a good prediction result because if there's none then he will just be lazy to do the research.

According to the OP, his friend observed that he needs more theoretical knowledge about gambling. There might be no book that will teach him how to win bets but his lack of relevant knowledge might have contributed to his losses. Discovering his lapses and working to improve his skills is not a bad idea.

I also didn't read books about gambling. Watching videos, reading game manuals and ToS of gambling platforms were my basic sources of gambling information. Joining gambling social media groups could be also helpful , since they usually give some helpful tips.

I would suggest books related to statistics and math. Gambling sometime is just "read the numbers" and undestand the meaning.
If you understand clearly these passages you will have everything easy.
I will not suggested any other book. There are funny stories about gambling, strategies and so on, but each player should find inspiration and experience by himself.

If we do really tend to look up on the range on where you would be needing to deal up with then you should be that focusing into the spot on which you do like to focus on. Lets say you are far more interested with sports betting then you should be that trying out to focus into it and of course on the moment that you do have plans on making up some betting then make it sure that you wont really be that betting up blindly and its recommended that you do really just that only make bets according or really that getting in line with your interest specially on a certain sport. There's no fun on betting on a sport that you dont even know, you will definitely be having that tons of loses because you dont really know on what it is in the first place.

Actually this is really that hard to make out some explanation and its much better that to tell your friend that he should be the one will be discovering it because we dont really know what are the things that we do really like to do then you will be that involving into tons of things on which we know that this  will commonly happen on a certain individual Just choose up on which one you would be liking and stick into that and also bare in mind that whether of these two then it is more important that you do really have that self control when it comes to spending money because this is where issues do really came up first and if you arent that serious with this one then gambling will really mess up your life if you wont be that careful.

There are so many books that they publish on how to get rich or become a billionaire but I don't know how many of them has worked practically on those who read them.

As for gambling books, even if there are guidelines from certain books, I don't believe they do help gamblers to start having a winning streak. Gambling doesn't have a generally accepted strategy. You have to build your own strategy and enjoy it with fun because gambling itself is about having fun with your money. So the more relaxed you are, the better you could make decisions.

To me, the least Op's friend could do is to learn from people who have made last tries and marks from gambling by taking a risk that later worked for them. People like Fred Smith who brought back FedEx with last $5,000 and he $27,000 by playing blackjack and that turned the company's fortune around.

So, he needs to know books don't matter in gambling but to taking reasonable risk as far as you can bear.

Yes that is how it works. In reading through the OPs post, I feel the friend believes he is missing out tricks to what would bring him winning.
So I think I the OP as an experienced gambler should just orientate the guy and bring it to his ideal that winning in gambling especially sport games are basically on lucky and not professionalism or some kind magical.

This guy need to be let known of responsible gambling so that he does not also put his mind up on adopting gambling as reliable source of income because his long time of gambling and no winning was what inspired him on what book to read just to catch up profits.