What is a good book for someone preparing for a grade 6 classical ballet exam?

You should try reading The Ballet Companion by Eliza Gaynor Minden. It has information on ballet's history, a glossary of terms, and lots of useful information on everything from doing a bun to auditioning and performing. What I expect would help the most, however, are the sections written on ballet movements like tendues, grande battements, and pirouettes. They have lots of pictures and give you really informative tips for each ballet movement!