Here are a few books and resources I have found helpful for understanding Bitcoin Core.

C++

Satoshi Nakamoto began work on the Bitcoin code in 2007/2008 using C++98. Bitcoin Core currently uses C++20. Here are some useful resources:

Online resources (all are up-to-date with the latest versions of C++)

• C++ reference
• C++ Core Guidelines by Bjarne Stroustrup and Herb Sutter
• Bitcoin Core Developer Notes
• Godbolt Compiler Explorer
• Google C++ Style Guide
• C++ FAQ

The Reference Book (unfortunately covers C++11, so a few parts are obsolete)

• The C++ Programming Language, 4th Edition (C++11)

Learning C++

• C++ Primer, 5th Edition (C++11, more gentle, longer, not to be confused with the knock-off, "C++ Primer Plus")
• Accelerated C++ (condensed, faster, quite out of date but excellent, exercise solutions here)

Intermediate/advanced (all but the last one predate C++17, however, and the last one is C++17)

• Effective C++
• Effective Modern C++
• Effective STL
• Exceptional C++
• More Exceptional C++
• C++ Templates: The Complete Guide (1st Edition)
• C++ Templates: The Complete Guide (2nd Edition)
• C++ Concurrency in Action, 2nd Edition

Cryptography

• Elliptic Curve Cryptography: a gentle introduction, a blog series by Andrea Corbellini
• Foundations of Cryptography
• Waxwing's blog by Adam Gibson
• A Graduate Course in Applied Cryptography by Dan Boneh
• Cryptopals - A set of cryptographic code challenges

Cheers,

Jon Atack