I can recommend 'Mastering Bitcoin' for developers out there, first chapters also great for anyone who wants deeper understanding. Here's a copy on the Tor network...
http://gd6is466quuhsgzbztyv4sjswdhgfii63wf54qsb32v27xmm5dxbtmid.onion/bitcoinbook/

...or if you want to read it on GitHub:
https://github.com/bitcoinbook/bitcoinbook/blob/develop/book.asciidoc