I have a bunch of useful resources, most of which are free:

- If you're looking for a concise (yet complete) guide: https://www.feistyduck.com/library/bulletproof-tls-guide/

- OpenSSL Cookbook is a free ebook: https://www.feistyduck.com/library/openssl-cookbook/

- SSL/TLS and PKI history: https://www.feistyduck.com/ssl-tls-and-pki-history/

- Newsletter: https://www.feistyduck.com/newsletter/

- If you're looking for something comprehensive and longer, try my book Bulletproof TLS and PKI: https://www.feistyduck.com/books/bulletproof-tls-and-pki/