We do a lot of traveling, and one of the biggest problems is the weight of guidebooks.
It's one thing to spend a long time in one major site, like London or Paris. A "normal" guidebook has enough detail to help, even if you've been there before and are down to "level four" sites, and the weight of one book isn't really a problem.
But, when you're visiting multiple locations, each for a short period of time, you can't really carry a complete book for each stop without needing a porter to help with your luggage.
So, when a friend who had been to Prague gave us his Prague Day by Day, we were sold and went out and purchased one for Vienna and Venice too.
While this series helps a lot in planning which sites to see on a short trip; has the basic info on transport, currency, power, etc; and includes the essential map - it's small enough to carry several and weigh no more than one full-sized book.