Hi, welcome to the forum!
There are a bunch of different directions you can go in, depending on your interests and experience.
bjc.edc.org is The Beauty and Joy of Computing, a yearlong secondary school course that starts at the beginning but moves in somewhat different directions from the SAP course.
Codierte Kunst by Joachim Wedekind is a fantastic book of Snap! programs that generate art in the style of various modern artists. It's in German, so I can't read the text, but the Snap! code is understandable. :~)
Structure and Interpretation of Computer Programs by Abelson and Sussman with Sussman is the best computer science book ever written. It comes from a former freshman CS course at MIT. It's available free online (click the link). It uses Scheme as the programming language, but if you're familiar with Snap! you'll find Scheme quite similar.
Parts of the Snap! Reference Manual are about aspects of the language that may be new to you, especially Section VIII, OOP with Procedures, and Section X, Continuations.
I'll put in a plug for my own Computer Science Logo Style trilogy, also free online at the link. It uses Logo, which is sort of the grandfather of Snap! (by way of Scratch), but it includes accessible introductions to advanced CS topics such as compilers and automata theory.