I found Mike Meyer's CompTIA A+ Certification Exam Guide to be of great help in passing the A+ certification. His seventh edition is written in a clear and concise manner that's easy to understand and also makes a great reference book.
However in some areas, for example, the CPU section, I think that Mike goes into too much detail for the A+ certification and I found there is not enough background or troubleshooting information on VOIP technologies. Even with these shortcomings, I would still highly recommend this book.