Humble Bundle have a DIY Electronics eBook bundle at the moment:
Covers Raspberry Pi, Arduino, Beaglebone & electronic circuits.
If you are not familiar with Humble Bundle, they package up deals and you pay what you want. Pay more than US$15 and you get 16 eBooks. Usually you can download in multiple formats, PDF, MOBI & EPUB, to suit desktop, iPad or eBook readers.
Proceeds are distributed between the publisher, charity and the Humble Bundle site. You can choose the allocation if you like.
It's worth checking every few weeks to see what they have.
Bundles are typically available for around 2 weeks each