- If you want to just make the simple GUI in Common Lisp, your best
choice, in my opinion, is LTK, which is a high-level interface to
Tcl/Tk scripting library
- There are plenty of other choices, but they require some
vision of what you are trying to achieve. If you want to use
one of the wide-spread GUI toolkits (GTK or Qt) there are CL-GTK2
and CommonQt. But you have to first of all understand well, how these
toolkits work
- LispWorks provides a very sophisticated GUI-builder library CAPI,
which is cross-platform
- On the Mac ClozureCL has good Cocoa bindings
- Finally there's a full-blown native CL solution for GUI - McCLIM -
which is very flexible, but pretty involved
Also, if you want to understand GUI and study it from the theoretical point of view, you should learn about various GUI-patterns, such as MVC, Model2, MVVM etc. I don't know about any GUI-framework in Lisp, implementing such patterns, so doing such a project may be an interesting learning experience...