The CLHS, or Common Lisp HyperSpec, is one of the most important resource for any Common Lisp developer. It is derived from the official Common Lisp standard, and contains documentation for every aspect of the language.
For this purpose, LispWorks provides an archive which can be downloaded and browsed offline.
While the HyperSpec is valuable on its own, the SLIME Emacs mode provides various functions to make it even more useful.
I have found the following functions particularily useful:
C-c C-d hto browse the documentation associated with a symbol.
C-c C-d ~, to lookup
C-c C-d g, to access terms in the glossary.
With the default configuration, Emacs will use the online HyperSpec. You can
have it use a local copy by setting
common-lisp-hyperspec-root to a file
URI. For example, if you downloaded the content of the CLHS archive to
(setq common-lisp-hyperspec-root (concat "file://" (expand-file-name "~/common-lisp/HyperSpec/")))
And if you configure Emacs to use the EWW web browser, you can work with the CLHS without leaving your editor.