I actually disagree with the first part. Decent musicians can keep a beat, yes, but when it comes to playing the drums, this is a whole different task entirely.
There is some great information in that theory for dummies book, and there are a lot of really great tutorials online. I would recommend using those first if you want an overview of what you already know, but if you can, I would also recommend getting a thick old theory workbook and just going for it!