[helij]

https://stellafane.org/tm/index.html has good guides. There are astronomy forums such as CloudyNights with their own ATM sections. Edmind Optics has All About Telescopes by Sam Brown available online - https://www.edmundoptics.com/ViewDocument/All%20About%20Tele...

There are so many books around this and many great website such as this one for example: https://www.bbastrodesigns.com/tm.html