Honestly, I've not seen hardly any books aimed at non-masons or for those who have not yet attained the degrees discussed. Considering that most discussion about the topics would likely make little sense at best or at worst do more to "spoil" the degrees than anything else, I can see why not many have been written. It is a novel concept, however, to write a book aimed at topics that will condition or prep the mind of the petitioner in such a way as to actually make sense and not spoil anything. For the man who is already a mason, but wishes to "read up" on further degrees, I would say again that you are doing little favors for yourself. If you want to know about the degrees, take them first and study them afterwards.
I realize that we are always taught to prepare for something by studying it before hand, but given that Freemasonry's lessons use the degrees as springboards for instructing them, it makes less sense to treat the lessons like that here.
I really can't think of many books that fit what is being asked for to be honest besides the usual answer of Complete Idiot's Guide and Dummies. My best and sincerest recommendation for the non mason is the books just mentioned and wait until receiving the degrees. A mind fully prepared to comprehend what is being said is necessary and your mind will be greatly prepared by actually receiving the degree and spending a great deal of time contemplating it and its lessons.
To the mason, I say read only as far as the degree you have taken. Freemasonry cannot be grasped by reading one book's/author's explanation. The meanings have several layers and levels of depth. You will be devoting an entire lifetime to fully grasp them. Endeavor to learn from your instructor and other brethren for your first 3 degrees. Afterwards and if/when you branch out to appendant bodies, still keep up with learning from brethren but expand your learning and supplement it with commentaries and books containing other mason's views and interpretations. There is very little that has one single meaning and much of what you learn's deeper secrets will largely depend on yourself.
I know this isn't really the answer that most want. It would seem a lot of new masons (myself included at the time) wanted some form of learning to prepare themselves. However, speculative masonry, like operative masonry, builds upon a foundation and layer upon layer. By constructing layers of understanding and mentally and emotionally deconstructing them again and again, will you really grasp what is being taught.
EDIT: However, to answer Browncoat's question a tad better, once you actually join SR, assuming you are in the SMJ, you will likely receive A Bridge to Light and Morals and Dogma. Couple these with the SR Monitor and Clausen's Commentaries and you'll have plenty to peruse on the SR. For the C&C portions of YR, there is Book of the Chapter and Cryptic Council, both by Mackey, these are fairly light but provide deeper insight on the Chapter and Council. Also Duncan's monitor provides some good Royal Arch insight. I'm not yet in the Commandery so I cannot comment there.
-JTM