Lisp from Nothing and Lisp System Implementation from [1].

There are quite a few open source Lisp implementations in addition to SBCL that you can peruse [2] (follow links to CL's supported by Quicklisp). If you are targeting a specific platform (JVM or embedded), then ABCL/ECL; see Clasp for using LLVM.

To reuse modules for writing your own lisp, see SICL [3]

[1] http://t3x.org

[2] https://www.quicklisp.org/beta/

[3] https://github.com/robert-strandh/SICL