I love old guidebooks and have found much entertaining and valuable information in them. If I need hotel and restaurant recommendations, of course the internet is the source -- not even new-ish paper guidebooks are much worthy.

But for depth of information, specially unexpected, these books are great.

I am talking, of course, of books printed at least before the 1950s. 19th century and early 20th are my favorites. I am not sure this is what you have in mind.

On the other hand, I particularly like reading about the impressions of great men when visiting great places, even if it is not a guidebook. Balzac or even Dumas can show us more of Paris than most guidebooks.