I sometimes get asked by law students or young associates whether I could give them a reference that might help them make an easier transition to law firm life. There are three books that I recommend - I'll identify them below and discuss their pros and cons.
1) What Law School Doesn't Teach You But You Really Need To Know
This 593-page monster offers the widest range of knowledge on a whole host of subjects in one book. Although a few parts may feel a little dated (it was published in 2000) and some other parts may feel like they are just "common sense," this book really tries to fill in as many of the gaps as possible for readers of varying levels. (That is, what is common sense to you may not have been common sense for someone else.) If you are really starting from scratch, then read it page-by-page. Conversely, if you already have a basic knowledge, then skim until you reach something that is new to you.
2) The Young Lawyer's Jungle Book
While the first book above primarily gives you a checklist of things to do or not do, this book focuses much more on the "feel" of law. That is, how you are expected to think and feel and what those around you are thinking and feeling, including partners. Recognize that knowing the unstated motivations of a senior partner can sometimes help you react in a more advantageus way - and that the partner's motivations are often different from what you have been told to expect and are often not at all transparent. This can be especially useful in practice because no matter how big the checklist, at some point you will have to start winging it - and you really can't wing it effectively unless you know what motivates people and what they expect of you. This book was written in 1999, and also sometimes feels a little dated, but here the dated-ness has a minimal impact on the underlying feeling. For example, the fact that somone would be talking to their friends on Facebook rather than on the phone is a technological difference between 1999 and 2009, but how that person feels is pretty much the same. Also, what they want to feel from you is also pretty much the same.
3) The Curmedgeon's Guide To Practicing Law
This is the most recent of the books that I like. It does not have the huge coverage of the first book, nor the detailed personality insight of the second book, but it is the most up to date and does provide good listings of things to do or not do and gives some insight into the mind of the partner. Unfortunately, it is pretty short.
Between these three books, you should have a good base to build on with regard to what is expected of you and how to act at the firm, as well as some understanding of those around you. Most people that I have recommened these books to have reported back that they found them to be very helpful - I hope that they are helpful to you.