I have been impressend and pleased at the steady increase in Kindle books. It grows by about 500 titles a day, or more. I expect this to continue at about that pace. Note, tthat Sci fi and fantasy have always been heavily represented in e-books (see Baen).
Adding back titles more than a few years old will be problematic, I think. Nowadays, most books are written electronically (on the computer), transferred to the editors and publishers that way, and probably rarely see print before the first galley proofs. Putting them into an e-book format, when they are already in a computer text format, would seem to be relatively easy and cheap. (But, apparently, not without problems as Jim Butcher's recent novel indicates). Older books may no longer be in a computer format (either not saved or never were), and need to be converted by scanning and OCR. This is a slower and more labor intensive project. I'm not all that convinced that the publishers will do it for older titles (even by popular authors like Cussler), since most buyers (like me) will be looking for a low price since you can get them in paperback for pennies (even on Amazon + shipping). I do hope that they will address those older books, but I do see problems there, as I said.