Solutions to excercises from the Structure and Interpretation of Computer Programs
Excercise solutions are labelled like "EX 1.21". Certain excercises are hard to illustrate, because they require modifying previous answers.
You can find the book for free here: https://mitpress.mit.edu/sites/default/files/sicp/full-text/book/book.html
You can also find a community wiki with solutions here: http://community.schemewiki.org/?SICP-Solutions
Finally, you can find some good lectures here: