I went to Barcelona recently, and in the planning of the trip I consulted four guide books:
Lonely Planet's Barcelona City Guide
Dorling Kindersley Eyewitness Barcelona and Catalonia
Cadogan Guides Barcelona (Dana Facaros and Michael Pauls)
Time Out Barcelona
All of them were good. I took two with me: Lonely Planet and Eyewitness.
The reasons I chose those two:
Eyewitness unfailingly has a good quality map, which in my view is LP's downfall. Eyewitness always includes a Street Finder Index which LP does not, and LP often misses out on streets other than the major ones, which can be very frustrating, especially in a city like Barca where there are many alleyways. I never use Eyewitness for hotels - the information is scant and usually more expensive than my (more Lonely Planet) budget, but it has fantastic colour photos and cutaways of buildings, and illustrations which place buildings within their streetscapes. It's a nice book to read on the bus or train going somewhere.
LP specialises in listings, which seemed to be more up-to-date and comprehensive than either Time Out or Cadogan, It also had a good Excursions section which helped a couple of times when I travelled beyond the city. I also liked a couple of the walking routes they recommended. Information on matters such as public transport is comprehensive and detailed. I like the chapters on history, architecture and food as well.
Time Out's great strength for me was in helping to plan. It had a huge array of hotels, and if you were especially interested in nightlife, I would take Time Out. I was there primarily for a conference, and many of my evenings were organised. And I'm also night a great nightclubber, perhaps contrary to Barca lifestyle!! If you are - consider Time Out. Its maps were pretty good, but not comprehensive for the area I was staying in, beachside Barceloneta.
That is where the Cadogan guide excelled. Its maps were so good that I pulled them out of the book and took them with me. I would have taken the whole book, but for the weight in my suitcase! Its info about places and sights seemed accurate and comprehensive.