For me it is<p>Handbook of Applied Cryptography
by Alfred J. Menezes, Paul C. van Oorschot and Scott A. Vanstone
<a href="https://cacr.uwaterloo.ca/hac/" rel="nofollow">https://cacr.uwaterloo.ca/hac/</a>
(link has all chapters available for free under permission from CRC Press)
Not a book, but Dan Boneh's Crypto course on Coursera is rad. Really enjoyed it and learned a lot.<p>(Note: My job is not related to cryptography in any way.)