The book is wonderful to read and get your mind blown by all the ingenious, clever, and surprising things that are possible using basic building blocks of Common Lisp - so that you know exactly what code you should NOT be writing when doing Lisp for anything serious and involving other programmers.

It's very good for realizing the difference between what is possible and what is feasible, since Lisp mastery requires both knowledge of what is possible to do and restraint in the actual use of those possibilities. Clever things require cleverer debugging techniques, and Let Over Lambda is absolutely full of the former.