I don't think it matters how little the client is paying for the advert – most designers would surely want to make the best of it that they can (within reason). So the following books are hopefully good starting points for designers who want to learn about advertising/writing – and I've found them really useful myself.
1. The Advertising Concept Book
This is a superb book which is as impressive in its design as it is in content. Every example advert (there is at least one per page) has been re-drawn in pencil – demonstrating one of the main points in the book . . that every idea starts with a pencil and paper.
2. Advertising Now! Print
Full of really creative adverts – this one will be useful to have open while you're reading the book above. Plus, there's an article inside it that I've included below – it's a really clever piece of writing that includes some good information about copywriting.
3. Writing that works
Although not directly linked to advertising, this book will definitely give you something to think about. One simple and seemingly obvious point from the book is: people don't care what you have to say about yourself, but they'll listen if you talk about them.