Yeah, I didn't know that someone wrote a book. I think that's just a "fanfic" thing where someone took the idea and published their own internet book.
The St. Rappeo story is simple. No Laying Up (golf website) does a YouTube show called "Strapped". Two guys, $500 budget for three rounds of golf, food, and accommodations. I consume new Strapped episodes like I used to consume the college football preseason magazines that hit the shelves in July.
In one episode of Strapped a guy named Ben joined Neil and Randy for a round in Baton Rouge. Ben made them custom ball-markers by stamping the word "Strapped" onto metal ball-markers. Neil looked at his and, seeing a space between ST and RAPPED, and reading the D as an O, said "St. Rappeo? Who is that?"
From that has grown an entire meme. Worried about water left on the 17th fairway? Pray to St. Rappeo.