Manning
As I bought yet another book few days ago, I noticed that most of my IT book purchases are going to Manning. Whenever I need a book they are first site that I check. If there is book there, I will check no further.
Probably main reason for this is their stance toward DRM - there is none! Once I buy PDF book I can read it on whichever device I want to. I can make million of copies. Only thing they add is your name on footer of every page. Cynics would call this social DRM but I find this acceptable and quite unobtrusive when reading on computer or paper.
Most e-book readers are notoriously lousy when it comes to PDF and Manning fortunately recognized this as of late. Almost all new books are coming with mobi (Kindle) and ePub support alongside PDF (and you only pay once for all three). Some older books are getting e-book versions also, but Manning is taking their time.
Selection of books is quite nice, especially in .NET compartment where you can find gem as C# in Depth by Jon Skeet, easily the best book on C#. Other authors and books are also way above minimum quality. I would definitely say that, at least for books that I have read, quality ranges from good to excellent.
If you want to get view of newest technology where book is just in writing, Manning has solution for that also. It is called MEAP (Manning Early Access Program) and, in nutshell, it enables you book access as it is being written. You buy book in advance and every month or so you get next few chapters to read and that goes on until book is completed. During that time you have access to author and you can give him notice of error, steer him in other direction, or just annoy the hell out of him. :) Rarely it happens that MEAP book is cancelled and in that case Manning offers full refund (or two free books for your emotional suffering).
Nice touch is that I probably never paid full price for a book. It is really hard to find time in year when there isn’t some coupon code to take 30% or more from purchase (they adore giving 42% discounts). If you are patient enough there is also Deal of the Day offering where each day there is a book with discount. I find it almost impossible to pay full price.
There are bad things also. Site looks like something from 10 years ago, their account center is almost impossible to find and, once you find it, you will curse at annoying interface. Other than this, they are pretty close to being perfect.
P.S. For company with colorful book covers, they have a really ugly logo.
P.P.S. No, I was not paid for promoting them.