Hi I am writing a book and the title is good to be "Coaches guide to football" but i'm not sure which is the correct use. Should it be "coaches' guide to football" coaches guide to football" or "coach's guide to football" as I have seen similarly titled books use a variance of all three.
Yes, all three are possible. I suggest The/A Coach's Guide to Football.

Coach=Singular e.g. one coach

Coach’s=Singular possessive

Coaches=plural e.g. more than one coach

Coaches’=plural possessive