For most "popular" books, the first edition is done in a hard cover with very good paper. About a year later a trade paperback will come out (often about the same size as the original, but on cheaper paper and with a paper cover). Finally, a true mass-market paperback will come along on slightly better than newsprint paper and in a smaller format.
Of course, popular books will often get picked up by book clubs which will often produce a special version (often hardcover, but maybe on cheaper paper, maybe without a dust jacket, etc.)
Finally, Amazon's supply and pricing policies are surprisingly complicated. I'm in book publishing and I'm no longer surprised when I talk to others in the field who don't understand it. All our books are sold by Amazon, but Amazon doesn't stock all our books. The ones it does stock tend to be our newer, more popular titles and they often discount these even more than we can on our own bookstore site--they have huge volume, so can get away with a very low margin. Amazon makes many books "available" but they aren't really in stock--Amazon is actually acting as a broker.
The book industry is a strange mix of high-tech and medieval customs!
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