hey man, and everyone else on this forum.
it takes years literally to write a book. I started mine two years ago and i'm not even half way...(but then it's a triology....anyways)
I say go for it, start now, gather everything you've written, everything you know.
make notes, remake notes, interview star players, interview coaches, interview doctors, interview newbies who you taught, watch games, analyse games, write, rewrite, draft, redraft, read, reread, distribute it on EF to ask for opinions, edit, redistrbute, ask for more opinions, reedit...find publisher, convince publisher, find another publisher, convince another pubclisher, visit around 5 publishers, find one and hope they'll take u on board....
i say start now...you'll gain a lot more experience this way too, because you're actively pursueing.
good luck, and i'll buy a copy too.