I bought the above book from Amazon a few weeks ago and I haven't been disappointed. It does just what it says, but this is far from a book full of writing prompts. Yes, the writing could only take five minutes but most of the exercises I have tried have led me onto larger projects and taught me something new about style. The best thing about the book for me is not only that you can simply open the book at any page and begin, but that you can repeat the lesson as many times as you want and create something totally new.
Even if I already have something in mind for my writing session, I have been starting with a page from this book. Topics range from thinking up a character's name to humanising nasty characters. Highly recommended.