I found Mike Myers' A+ Certification manual to be the easiest to read all around manual that will give you basic knowledge of computers. He starts with simple analogies, and works through up to modern computers and their internal parts. Even if you have no interest in getting an A+ certification.
i recommend Mike Myers' CompTIA A+ book it comes with a CD that you can take practice test through.
Edit: The new test does not cover Windows 7, while the book mentions this, it still does go over some of the basics of 7.
Read This If you really want to pass it, study. Read a chapter a day (usually about 50 pages, should take you a month to read the whole thing) then go take the test. The A+ Exam is stupid as hell, it doesn't matter how long you've been repairing computers, 2 weeks or 20 years, they are going to ask you stupid questions that you will NEVER run into in the real world.