Authorized (they pay me royalties) electronic copies of my books (including Tor titles) are available at Baen Ebooks (http://www.BaenEbooks.com). Baen’s Bar has two discussion areas where you can get more information about e-book formats for various readers: Webscriptions EBooks and EBook Reader.
My available e-book titles are listed at http://www.baenebooks.com/s-32-david-drake.aspx. Instructions for downloading for the Kindle, the Nook, and other devices are at http://www.baenebooks.com/t-ereaderinstructions.aspx.
As of December 2012 my kindle books are also available at amazon.com.