It might be useful to learn Common Lisp because there are excellent free books available online and nearly everything you learn will be directly applicable to Arc. I've been reading On Lisp to augment my understanding of Arc. Believe me, the knowledge gained by studying another Lisp dialect (or an "unfinished" version of Arc) will not become obsolete.
On Lisp: http://paulgraham.com/onlisptext.html
Practical Common Lisp: http://gigamonkeys.com/book/
