I do not buy a (regular) guidebook (e.g. Lonley Planet, Vis-A-Vis., etc.) older than year, better six month. Meanwhile I know where the publishing companies hide the date of publishing (they have tp print it in the book in Germany).
I try to reconfirm the information in the book before I start my trip by searching the internet (e.g. local homepages, Tourist info, Flyertalk) and perhaps print it or copy it to my laptop. I know, this means I could serach the web instead of buying a book, but I do not discover a city with my laptop on my knees ...

If I buy a guide about cultur (e.g. art and history, architecture) with only aa few touristical information in it, it could be older. These books are not so often updates like the typical guidebook.