SBCL is a mostly-conforming implementation of the ANSI Common Lisp standard. This manual focuses on behavior which is specific to SBCL, not on behavior which is common to all implementations of ANSI Common Lisp.
Regardless of your ability level, two very useful resources for working with any implementation of Common Lisp are the ILISP package for Emacs and the Common Lisp HyperSpec.
If you're not a programmer and you're trying to learn, many introductory Lisp books are available. However, we don't have any standout favorites. If you can't decide, try checking the Usenet comp.lang.lisp FAQ for recent recommendations.
If you are an experienced programmer in other languages but need to learn about Lisp, three books stand out.
ANSI Common Lisp, by Paul Graham, will teach you about most of the language. (And later it might also be worth checking out On Lisp, by the same author.)
Paradigms Of Artificial Intelligence Programming, by Peter Norvig, also has some good information on general Common Lisp programming, and many nontrivial examples. Whether or not your work is AI, it's a very good book to look at.
Neither of the books above emphasizes CLOS, but Object-Oriented Programming In Common Lisp by Sonya Keene does. Even if you're very knowledgeable about object oriented programming in the abstract, it's worth looking at this book if you want to do any OO in Common Lisp. Some abstractions in CLOS (especially multiple dispatch) go beyond anything you'll see in most OO systems, and there are a number of lesser differences as well. This book tends to help with the culture shock.