I was browsing at Amazon and noticed that for many of the RS books, they also say when the update is due.
Posted by
SE Pennsylvania
2730 posts
That info is available below the description of each guidebook here in the RS store, too.
Posted by
Irving
3316 posts
Both RS and Lonely Planet websites say when the next edition will be available.
Posted by
Boise, Idaho
1258 posts
And errata and critical updates/corrections are available as a pdf for each book. Look at the bottom of each listing for "Updates and Feedback."