Richard --
As far as I know, there is no book that goes into depth about writing
formulas in custom fields in Microsoft Project. The reference that I
personally use, over and over again, is the system of Help articles within
Microsoft Project. If I do not remember how to use a particular Function, I
simply read the Help article on that Function. Beyond this tip, I would
also recommend that if you need help writing a specific formula or
understanding how to use a particular function, then simply state your needs
in a post in this newsgroup and we will try to help. Hope this helps.