I'm currently trying to get proficient in Common Lisp and to learn some of the tricks for writing compact, clear and beautiful code in it.
So, I want to know if you have any sources of good Common Lisp, preferably free and online but books are also OK.
Most open source Common Lisp (CL) environments ship with a lot of CL source code.
Take a look at CMUCL, CLISP and SBCL.
Cliki (the Common Lisp Wiki) has many open source CL packages.
Google Code Search is another large repository of CL code. Setting the language to "Lisp" will return both CL and non-CL files, like Emacs Lisp (.el) files. To narrow the results, set the Files text box to .lisp$ so only file names that end in .lisp are returned.
Paradigms of AI Programming. The code is online, though the book itself is not to be missed.
On Lisp is also very fine.
Large codebases can be schizophrenic, because there are often many contributors to the project. I would also say that contributors tend to want to add new features rather than re-write some code because it could be coded in a slightly more elegant way.
Paul Graham is attributed with good coding style. The link points to pages where his coding style in ANSI Common Lisp is commented upon.
Peter Norvig has also written about good Lisp coding style here.
The most admirable modern Common Lisp I've seen is in Edi Weitz's libraries. (Look within the outline area NerdStuff/Common Lisp/Code available on this server.) His CL-PPCRE library is worth studying in depth.