O'Reilly's Safari service is popular because it lets you read tech books online for a small monthly fee. You get a 'bookshelf' with 5/10/20 slots (or whatever level you pay for) and you keep books on your bookshelf for 30 days while you read them. Each 30 days you can change the books and read more if you wish.

Safari was already good, but O'Reilly have very quietly launched Safari Library, a $39.99 per month service that lets you read any book on the Safari service (counting into the thousands). There's no more bookshelf, so you can read any book at any time. You basically have a large, official, professionally written compendium covering every computer technology you'd care to read about. Also, if you sign up now, you 'lock in' the $39.99 per month rate for life. I'm considering it.