I recommend Security without Obscurity: A Guide to PKI Operations by W. Clay Epstein and Bulletproof TLS and PKI by Ivan Ristić.

I started working in this space a year ago (I'm on a project deploying zero trust networking at a large company) and these books have been invaluable.

https://www.amazon.com/gp/product/036765864X

https://www.feistyduck.com/books/bulletproof-tls-and-pki/