On Friday I shipped the last two copies of The Mythic Seas (AG0259) from our warehouse to a distributor. Warehouse 23 still has some copies, last I checked, and there are copies out "in the channel" (on distributor and retailer shelves), but we have no more copies to restock them when they sell through. So if you're interested in a printed book, don't hesitate if you see a copy!
In the meantime, the book is now available on RPGNow.com as a pay-download PDF, for those who are content with content sans dead trees.
(We do not plan to reprint the book, nor are there any plans to revise it for 5th edition, at least as far out as we've done our rough planning.)