If you are looking for a comprehensive guide to patent law that is both approachable, understandable, easy to read and thorough you have really only one option and that it this book, An Introduction to Patent Law, by Professor Janice Mueller of the University of Pittsburgh College of Law.

I have taught Patent Law myself for years, and I always recommend this book to students.  It is not written as a treatise, but rather is written as a self contained guide to patent law that can be used as an exceptional study aide for law students, or as a desk reference for inventors.  While there are other books out there that are less expensive, the price you pay for this book is well worth the investment.  This book covers a wide range of topics, including patent claims, the patentability requirements (i.e., utlity, patentable subject matter, novelty, nonobviousness and the description requirement), patent damages, patent remedies, patent litigation theories and defenses, and international patent issues.

It has always been amazing to me how so much useful information could be put into a single book without compromising on quality.  It is sadly true that many books that try to cover so much ground wind up giving only brief and inadequate treatment of the issues.  That is not the case here and I am confident that anyone who adds this book to their collection will find it to be useful and a great price for what you receive.