This recent little book is a gem: it`s Mick Napier`s personal take on what Improv is and how it should/could be done.
The book is not for absolute beginners. Napier assumes a basic knowledge of and/or experience with Improv, and dives right in with a set of rules, only to conclude that the rules are irrelevant.
You may or may not agree with Napier`s take on things, but as he states himself somewhere in the book, there are various approaches to Improv, and none of them are right and none of them are wrong: every approach has helped someone at some time improve their work. Napier`s approach will probably improve yours as well.