There are two great problems when learning Arc: the lack of documentation and the quite cryptic error messages. For these two reasons I would reccomend to know a little lisp before learning Arc. A really good book is Practical Common Lisp (http://www.gigamonkeys.com/book/). I suggest reading at least the first ten chapters.
If you want to learn Scheme first try How to Design Programs or Structure and Interpretation of Computer Programs (both free online). HtDP is more beginner oriented, so if you can program I recommend SICP. There are valuable lessons in HtDP even if you are a programmer, but it's not as challenging as SICP so HtDP might be boring.
