SBCL's user manual is available on the web for the latest release in HTML and PDF formats. See the contrib/sb-manual/doc/ directory for the current sources. The man page is at doc/sbcl.1, while the Info pages are bundled in binary distributions as doc/manual/sbcl.info*. For how to compile the documentation yourself, see INSTALL.
Also, visit fixnum.com for an unofficial rendering of the user manual at the current tip of the master branch in HTML, PDF, Markdown and plain text formats. These versions are easier to navigate (they have a lot more internal links and are also linked to the Hyperspec), and they also include the default values of arguments and the initial values of variables.
See also the SBCL Internals Manual, which describes SBCL's implementation.