Scott Helme, creator of securityheaders.io, wrote a good introduction to certificate transparency.

Certificate Transparency is an open framework for monitoring and auditing the certificates issued by Certificate Authorities in near real-time. By requiring a CA to log all certificates they generate, site owners can quickly identify mis-issued certificates and it becomes much easier to detect a rogue CA.

https://scotthelme.co.uk/certificate-transparency-an-introduction/

Oh Dear!, my side project leverages certificate transparency logs to send you a notification as soon as a new certificate is issued for your domain.