It depends, what you mean by complex scripts
You can find for P2PKH and P2SH easily Information, and also for simple multisig. But at the point where a simple ‚if condition‘ comes into play, the sources dry out. Best I have seen here in the forum was from CIYAM here:
https://bitcointalk.org/index.php?topic=1300723.0I haven‘t found much info elsewhere on the net, seems like the whole knowledge is captured in libraries, and no tutorial is available. So need to go the hard way ourself?
I replied a similar post here, with another link inside:
https://bitcoin.stackexchange.com/questions/74375/getting-started-with-script/75349#75349