Maverick for Sci-fi you can't beat Baen.
http://www.baen.com/library/For Public Domain books I prefer Manybooks.net over everything else.
http://manybooks.netSort by genre, author, titles, etc.
Once you find an author you like you can click on his name and get a complete list of all the books they have by that author. Click on a book, and it will take you to the download page for that book. Often have a short description, picture of the front cover, and a download link that lets you choose what format you want it in.
http://manybooks.net/authors/haggardh.htmlI'll use H Rider Haggard as an example. He wrote in the late 1800's and early 1900's most of which are set in England, Africa or both. He was the original Author for King Solomon's Mines. The list of possible file formats is extensive, but it does vary some from author to author. Manybooks get most of their books from Project Gutenberg, but its easier to find what I want at Manybooks.
Many Library's are converting to Ebooks.
http://wiki.mobileread.com/wiki/EBook_Lending_LibrariesMost library's tend to use Overdrive or Adobe DRM'd PDF formats.
So if your going to use a library as a source that will effect what reader your going to want to look at.
If you buy a Kindle or to a slightly lesser effect Sony reader, you are going to be at least somewhat tied to their store. Mostly because both of proprietary file types. Granted you can get some files converted for kindle, but I don't think they'll touch a DRM protected library book.
My wife is into the Mystery, Thriller type, and its a lot more work finding her books than it is finding me Sci-fi fantasy stuff. I've got Thousands of books stacked up waiting for me to find the time to read them. For my wife I have at best 3-400 good ones that she'll enjoy. Its a bit of a never ending task.
Again I'll say that if your interested at all in ebooks and ereaders spend some time reading over at the Mobileread forums.
http://wiki.mobileread.com/ Register for a BBS acct, lurk around for a bit. You'll find me posting once in a while in the general discussion, but quite often in the Jetbook forums. (GhostHawk) (yes they have separate forums for different devices, fewer fights)
In my opinion what reader you want to look at, is going to come down to where and how you want to buy your books, and what you can afford to spend for them.
If you want the convenience of buying a book through your device, (3g, wifi, etc) then the new 150$ Kindle would be a great place to start.
If you want a bit more control, a bit more flexibility then perhaps you should look at the sony line.
If you are just as happy reading Scifi-or old classics out of public domain, and want total control.
Then one of the Jetbook's would be your best bet.
Of all the readers out there Jetbook is the most flexible, in that it reads more formats than any other.
Including: DRM ePub, ADE ePub, ADE pdf, HTML, PRC, MOBI, RTF, TXT, PDF, FB2, (Images)JPG, GIF, PNG, & BMP
# Built-in translators for English, German, Polish, Russian, and Spanish
# Font zoom from 12pt to 32pt
# 5” TFT LCD screen
# SD card expandability up to 32GB
# Battery life for over 23 hours
Feel free to shoot me a PM if you have questions, or would like to know where I found those 20,000 + ebooks I have sitting on my hard drive. (over 10gig's, and more on the way)