Willy Wonka, the famous childrens book by Roald Dahl, is a very good idea for a book character costume. Mr Dahl has invented some fantastic childrens characters, and they regularly make a really good subject to base book character costume on. Some of the favourite costumes from the book include Willy Wonka himself, Charlie bucket, the poor boy from a poor family, or an Oompa Lumpa (see below!). It would be really good to go in a group, each going as a different character. You could have a few oompa loompas, Charlie, Willy, and the other main characters like Ed TV, Veronica Salt and Violet - for this last one you could go as a giant blueberry - as she turns into this during the tour!
Oompa Loopa Costume
There are loads of places to buy or order costumes like this - see the links in the side for ideas for good websites that sell book character costumes.