The Jeppesen Instrument Commercial Manual is a good read for covering all the topics on the instrument practical. I read it straight through before my instrument ride, but it's also good to use as a reference whenever you have questions about a particular subject.
Rod Machado's Instrument Pilot's Survival Manual is also a good book to read before you take the test. It is a more entertaining read than the jeppesen book and has a lot of humorous anecdotes peppered in to liven things up.
Besides those two, I used the ASA Oral Exam Guide, the FAA's instrument flying handbook, and the PTS.