The Society of Authors does have a booklet on - pause to look for it - Ghostwriting and Collaboration Contracts. Free to members, £5 to non-members. Knowing them, it'll have everything you need to know.
http://www.societyofauthors.net/soa/section_page.php4?hp_nav_id=16&urlsection=Publications
My instinctive cautions would be that, as well as making sure you get a reasonable rate or fee for the job, you need to agree what happens if the sales rocket way beyond what's expected, if subsidiary rights are sold, if -
per impossibile - it were filmed, and so on.
Emma