Do you want to learn the software?
Lynda.com will take you from knowing nothing to being able to fumble with design within a short period, then you can skip to certain tutorials when needed. It is worth the price.
Or are you looking to learn design?
This is where it gets sticky... you can go to Barnes and Nobles, but some have zip about design, or you might get the wrong books. They will not carry the books and Magazine I'm about to list
You need to learn layout, typography, color, and maybe logo design...
First, get a subscription to Signcraft...
Second, you might want to get the book Mastering Layout by Mike Stevens, it is pre computer, and very geeky on balance, more than one person has told me it was too much for them, but I still recommend it.
Third, get Dan Antonelli's logo design books, it's geared toward small business from the sign side of things.
There was a book about sign shop sign design that was decent, but I can no longer recommend it for other reasons, I can tell you the title and where to get it offline or through PM, but will no longer openly list that book.
i have around 1500-200 design books, so narrowing them down to what you want it difficult, if you have specific needsm I can recommend a few titles...