I would recommend getting a Common Lisp (nice free trial versions available from commercial vendors) and PG's ANSI Common Lisp book, second choice Peter Seibel's Practical Common Lisp. Ignore things in CL you do not see in Arc <g>. But the core ideas documented in solid CL references will help directly with Arc.
Scheme is a second choice, but if you catch fire and want to start programming in anger with some Lisp I think CL gets the edge. Just my opinion, of course.
