I recently discovered an absolutely wonderful old book (written in 1977) called Writing CREATIVELY by Joan D. Berbrich at our library book sale. It's a workbook, designed for a three-ring binder with perforated pages, though I will probably be copying them out and using them instead of writing in the book, since I like the book so much.
The book has a unique perspective on things and makes me really think about what it's saying. There are frequent exercises for brainstorming, word building, and observation skills. It's concise and funny at times. I had one of my most successful brainstorming sessions ever after reading the first chapter. It really explores the lesser-used aspects of writing not many modern writing books explore, which is one big reason I like it.
I'd be willing to post a sample, if anyone would like one.
The book is available fairly cheap on Amazon here: Writing CREATIVELY by Joan D. Berbrich