Like Hellmark, I am a fan of O'Reilly when it comes to technical books. I like the multiple format options and their no DRM policy.
I normally read novels, mysteries etc. on my e-reader and do not enjoy reading literature on computer screens, at all. On the other hand, when it comes to technical books with high table and graphic content, I much prefer downloading PDFs and reading them on the computer. With my text editor, application and/or web open and running I can apply what I'm learning as I read.
The nice thing about O'Reilly is I can download a PDF for my computer and then download an epub version for my reader, should I go somewhere and not want to drag a laptop with me. You can also make a bundle purchase and get both the printed version and digital versions if you like. The bundle price is generally five or six dollars higher than the printed only version price. Having hauled several hundred pounds of computer manuals to the local library over the years so I'm pretty much all digital all the time these days and for the technical side of things
O'Reilly does it for me.