Joan Rivers -- Costco Won't Sell My Book!

Joan Rivers' new book is set to hit stores next week, but you won't find it at your local Costco ... because Joan claims the bulk retailer has cancelled its order because Joan refused to remove a few raunchy jokes on the back cover.

Joan called in to TMZ this AM in the middle of the morning meeting and explained how Costco objected to 2 fake testimonials:

-- Wilt Chamberlain: "Even if I were alive I wouldn't f**k her."
-- Marie Antoinette: "I don't like her. Let her eat s**t."