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Packt is also worth considering, with a similar lack of DRM and multiple formats.

As a nice bonus, every day, they make one title available free - a good means to dip into new technologies just out of curiosity:

Packt is very hit or miss. Some titles are awesome, and others read like a collection of SO answers and out-of-date official documentation.

Yes, I have bought books from them in the past. Thanks.