While in town to play at the Bank of America Pavilion, Gregg Allman figured he’d try to sell some books. So the 64-year-old singer/keyboardist stopped by Brookline Booksmith Monday to sign copies of his entertaining new memoir, “My Cross to Bear.” The book, which he co-wrote with Alan Light , is a candid account of Allman’s life in music. (We’re betting former bandmate Dickey Betts doesn’t like the book.) If Allman has a little extra spring in his step these days, it might be because he’s engaged to be married — for a seventh time! — to 24-year-old girlfriend Shannon Williams.