Humble Bundle hast just announced a new cryptocurrency book bundle. The prices are low but all of the versions are digital (often a mix of .mobi, pub, and .pdf). A good number of the books are about Bitcoin, but there are a few general ones on blockchain, and one on Ethereum. The price is low for the entire bundle, only $15.
I've ordered books from here before and had no problem with access. You can access your bundle at anytime in the future from the website after you have purchased it.
I haven't read any of these titles but I'm likely to buy the bundle and potentially complete a few book reviews in the future.