Besides the obvious Big 4 (Gentle Intro, PAIP, On Lisp, and Cookbook) I am also quite fond of Lisp in Small Pieces. Aside from those I'd also recommend grabbing the GCL source and building the Info manual so you can browse it in Emacs, it contains a fairly complete copy of the HyperSpec (I know it's available online, but I don't like visiting websites that haven't updated to HTTPS yet).

Also check out CLiki (the Common Lisp wiki, https://www.cliki.net/), it's very helpful in finding useful libraries, like Alexandria, defstar, trivia, lparallel, and so on.