What Lisp should I buy?

AutoLisp is a small, dynamically scoped Lisp. It lacks modern Lisp features such as a macro system or let bindings. Aside from the core language, most of the primitive functions are for geometry or the manipulation of graphical entities in AutoCAD. The properties of these graphical entities are revealed to AutoLISP as association lists in which AutoCAD "group codes" are paired with values that indicate properties such as points, radii, colors, layers, linetypes, etc.