A lot of tech book publishers actually release their books from their own websites in DRM free formats like epub, such as Manning, No Starch Press and often O'Reilly if you get them from the right place (humblebundle.com generally is a pretty good source for that if you're patient.) Sadly O'Reilly's website has stopped selling books directly from their website and instead you have to get them from somewhere else (but before they were DRM free).