I'm a big fan of Stahl's Psychopharmacology. Since pretty much every illicit drug has a legal equivalent, but the legals are better studied, you'll probably find that a good source. If your primary interest is illicit drugs, though, I don't have a good suggestion - it's not the sort of book too many careers are built on.

Stahl's Essential Psychopharmacology is also free and online[1], and written in a way that people like myself can understand it.

edit: actually only sections of it are available online. The book isn't hard to find, though.

[1] https://stahlonline.cambridge.org/common_home.jsf

Awesome!

Thanks!