Recs.
Updated
(SICP for short) An online copy of the text is available for free here.
The authors' lectures are available on youtube.
Specs
Pros
Pro Knowledge can be applied to any language
This isn't a book you'll glean direct practical tidbits out of, an introduction to not only functional programming but how to think in a paradigm outside the usual way C*/Python/Ruby/Java/etc... are coded.
Even if you wind up never working in Scheme or any other primarily functional language, the tactics and thought processes you'll learn here will apply to any currently-evolving language to a greater or lesser degree. You'll be able to map your thought process into the paradigm that works best for your current situation and not just be forced into a limited set of idioms that causes unnecessary boilerplate and clunky code mangling.