I began learning Common Lisp (CL) from the Common Lisp HyperSpec (CLHS): <https://www.lispworks.com/documentation/HyperSpec/Front/Cont...>. When I began learning CL about two decades ago, I did know of any other easily available source, so CLHS was my only source back then and I think it has served me well.

A popular recommendation these days is Practical Common Lisp (by Peter Seibel): <https://gigamonkeys.com/book/>.