"There are subjects treated here that can be appreciated only if you make an effort proportional to their innate difficulty. To harken back to something like the language of courtly love in medieval France, there are certain objects of our affection that reveal their beauty and charm only when we make a chivalrous but determined assault on their defenses; they remain impregnable if we don't lay siege to the fortress of their inherent complexity. In that respect, the study of programming languages is a discipline that demands the mastery of tools, such as the lambda calculus and denotational semantics. While the design of this book will gradually take you from one topic to another in an orderly and logical way, it can't eliminate all effort on your part."
Lisp has some of the best literature around of any programming language. Anybody who really cares about craft of programming should make use of wisdom therein.