I really enjoyed both Rod Machado's private pilot handbook and his instrument flying book. There is a ton of info in the Private book but there are many color illustartions and jokes in the margins that make it easier to get through.
Besides that, the FAR/AIM is really your best study resource, as is any other official FAA publication relevant to what you're doing. I use the FAA's Pilot's handbook of Aeronautical Knowledge and the Airplane Flying Handbooks qutie often.
I also study the POH of my aircraft reguarly just for safety purposes and for oral/practical exams, I do my best to know everything in the ASA Oral Exam guides and the PTS. For writtens, Gleim has given me the best results (never scored less than a 98 when using gleim to prepare).
I prefer material that comes straight from the FAA because you can be pretty confident that you're learning things you need to know for your practical if it's in those publications. However, for ab initio type studying, a resource like Rod Machado is better because it puts things into plain language.