HTTPS is basically HTTP over TLS, so start with RFCs:

- TLS v1.2 (https://tools.ietf.org/html/rfc5246)

- Certificate validation (https://tools.ietf.org/html/rfc6960)

- PKI is a little more tricky, as there's no single standard that defines it, though to get basic idea of what are certificates read RFC 5280 (https://tools.ietf.org/html/rfc5280).

Sure, you can dig as deep as you want, but these resources should give you a good starting point.