the winbeckler book for figure piping is excellent (i think the faces of his people are kinda creepy-looking, but the animals are lovely -- i used to do the alligator a lot and it was always a hit)...it's got excellent instructions and is easy to follow. a co-worker of mine stole my copy a number of years ago (my mistake for leaving it in the workspace we shared, but what a b*tch!)
your local library (and the other libraries in your region which will loan books to your local branch) can be a wonderful and inexpensive (free) resource for decorating books. when i would visit my parents in florida around christmas time, i used to come home from the libraries with my mum (who's a library junkie) every few days with arm-loads of decorating books.
my mum got all my vintage wilton books (when they used to actually have huge recipe and instructional sections) at garage sales in florida.
my stepfather will be auctioning my whole American Cake Decorating Magazine collection on ebay soon; ebay can be great for finding back issues of magazines and out of print books; just be sure to consider the shipping costs in your budget -- you don't want to be spending too much.