in my opinion it really depends on what you mean by "blockchain developer". nowadays i see a lot of people who are just interested in creating some nonsense token on one of these platforms (eg. ICOs) and call themselves "developers". for that you don't really need to learn much, or even learn blockchain technology for that matter.
but a real "blockchain developer" is someone who is familiar with cryptopgraphy, programming and P2P protocol. each of these are complicated topics that take a lot of time to master and depending on your end goals you may want to focus on parts of it more than others.
and these skills aren't going to be gained by reading books. you need to actually look at some code and write some code yourself to learn them.