A definitely good book about the show is Sylvia Stoddard's Inside Gilligan's Island. Among other revelations, it tells where Gilligan and Skipper met (the U.S. Navy.)
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"The Skipper (to Mr. Howell): "You would have to pick a couple of losers."

Sylvia Stoddard did not write Inside Gilligan's Island. Sherwood Schwartz wrote Inside Gilligan's Island. Sylvia Stoddard wrote TV Treasures a Companion Guide to Gilligan's Island. Both of those books are good. Another good Gilligan book is Gilligan Maynard and Me by Bob Denver. Also Gilligan Unbound is good. Actualy any book that mentions Gilligan's Island is good. Joey Greens unofical Gilligan's Island Handbook is great but is very hard to find. The only GIlligan books I have not read are Dawn Wells Gilligan cookbook and Russell Johnsons book. I would like to someday I have just never found them. I think you can buy Russell Johnsons book from his site.