The 30-year-old man – who has not been named – was spotted acting strangely at Crawley Library, West Sussex and approached by security guards who found him reading the book while he covered his lap with a coat.
A source said: "He spent about 20 minutes looking through the books in the business section before selecting Alan Sugar's autobiography.
"He was behaving rather oddly so we kept an eye on him and then he just sat down with his coat over his lap and started pleasuring himself as he looked through the book.
"It was very bizarre behavior and our security guys got a hold of him before too many people saw what he was doing."
Police were called and held the man on suspicion of outraging public decency.
