I looked up a few authors I'm interested in, to see if Kindle editions are available yet.
Heinlein - same 5 books available everywhere else.
McCaffrey - looks like a pretty complete lineup - 50 items, same as Fictionwise.
Orson Scott Card - only 4 books, same as Fictionwise.
Garth Nix - 4 books, all available elsewhere - no "Keys to the Kingdom" volumes
Tamora Pierce - only 3 books, same as Fictionwise
Terry Pratchett - 34 titles (No
Good Omens!)
Neil Gaiman - 16 books, still no
Good Omens
Shannon Hale - none - a "children's" author who has won multiple awards
J.K. Rowling - none (no surprise there)
E. Nesbit - 8 books, presumably all from PG
Steven Brust - none
Tolkein - none
Paolini - both books plus the "limited edition" of Eldest -- what kind of sense does that make???
Asimov - 25 titles, including the Oct/Nov 2003 issue of
Asimov SF Magazine
???
P.C. Hodgell - none, good thing Baen has them
Charles de Lint - 6 titles (ok, FW only has 2, Mobipocket has 3 - Kindle is ahead here.)
Jacqueline Wilson - two, not her best (IMHO), same as FW
Meg Cabot - 37 books, some of which say "(MOBI)" in the title
- FW has 38
Spider Robinson - none (FW also has none, Baen has only a few)
C.J. Cherryh - 27 books, reasonable coverage, but no
Wave Without a Shore - which has just become available via Fictionwise (bringing their total to 28). Woohoo! Guess who gets my money today!
Overall summary: Amazon may boast the largest inventory of e-books, but they don't have significantly more of what I'm looking for than existing vendors... yet. Except for Charles de Lint, who's hard to find in ebooks even on the darknet. So that's something to watch.