The Baen Library rocks. They don't have iSilo format by default, though, but it's easy enough to convert the html versions.
I should say that Baen has other e-Book offerings, too. The WebScriptions program on their site gets you electronic versions of newer books which you can purchase individually, or receive as part of a monthly subscription package. All of their content is distributed unencrypted in several reader formats, including HTML, which can be converted to anything.
Also, lately, some Baen hardcover books have been coming with CD-Roms which include selections both from the Free Library, and from Webscriptions. I got War of Honor by David Weber last year, and it came with ALL of the rest of the Honor Harrington books on the CD-Rom. The license agreement said "This disk and it's contents may be copied and shared but NOT sold." I take that to mean I'm allowed to do
this.
Enjoy.