These four eBooks are The Book Of Satoshi: The Collected Writings of Bitcoin Creator Satoshi Nakamoto, Bitcoin 123 by Phillip Caldwell, Bitcoin Guide For Beginners by Dale Blake and Bitcoins: How to Invest, Buy and Sell by Kinsley Wallace.
Please bear in mind that the books are not free. But on the other hand they don’t const a lot and are in my opinion a very good value. I am aware that there are many free Bitcoin related resources on the web but they mostly reiterate the same old and boring stuff. These book offer you a fresh and original content which has its merit and value.
I wish you all pleasant reading and may you learn something new and valuable.