The Right Word is the story of Peter Roget - the author of the original Thesaurus. It's a hardcover and costs around €15, more than I usually spend but I indulged because firstly, I always LOVED my Thesaurus and secondly, my kids have spoken about Action Jackson (the picture book story about Jackson Pollack) many times since we got it from the library.
The Right Word is one of those books whose target audience is really older than you would first think. It's a picture book, but is telling a story that is for kids older than the usual picture book readers. (I'd put this at ages two to five) What children need for this book is a familiarity with what both a dictionary a biography are. This will in turn lead to questions like "So he really did this?" and you can say "Yes!" and it all gets even more interesting for them.
Not that this book really needs any help to make it interesting. It's very nicely written and has lots of detail in the illustrations - actually it's sort of Oliver Jeffers-ish with lists of words and thought bubbles. Look below - it's lovely.
So, on the one hand, it's pretty pricey, but on the other it has a good shelf life and is an excellent book to have while your kids are in that "brains like sponges" phase. There is a big trend for these quality biographies for kids at the moment and I love it. I think next on my list will be The Noisy Paintbox.