Diamond Comics is listing THE BEST OF SIMON & KIRBY hardcover (DEC084249) as shipping this week. From experience, I can tell you that books like this sometimes have a several-week-long roll-out as different warehouses receive and ship it out across the country, so check with your vendor of choice before you make a special trip down for it.
Amazon is still listing May 12, but presumably they and other mail-order places will start shipping out orders as soon as they actually have the book.
looking forward for simon and kirby…thanks