Both the RS Barcelona guidebook and the Runner Bean tours website recommend the walking tour on the Spanish Civil War given by Nick Lloyd.
http://iberianature.com/barcelona/history-of-barcelona/spanish-civil-war-tour-in-barcelona/
I took the tour on the day after Easter this spring, my last full day of a two-week vacation in Catalunya. It was the crescendo of my trip. Nick Lloyd is energetic, passionate, and expert in all things related to the war, not just the facts and figures but the background and the consequences as well. This tour fulfilled my highest expectations for making travel a life-changing experience.
On your own there are things you miss no matter how well you prepare for a trip. Having a live guide who knows his subject, his city, and how to convey what needs conveying to a particular traveler colors and brightens the very same streets (the ramblas, for instance) that you might have walked before. Nick made a point at the beginning of the tour of asking us what brought us to him -- what is our interest in the war and in Barcelona. In our group there were clearly some college lads intoxicated by the idea of revolution -- Nick made sure they heard and saw a little of the reality of the infighting amongst the rebels and the excesses of the activists. Everyone brings along their baggage, and Nick had the teacher's knack for using what we knew to build up and expand our understanding of the period, and how central the war indeed was to all of European politics.
If you are a history buff or have an interest in leftist politics, you should include this tour in your Barcelona itinerary. Note that it lasts about four hours, and there are a couple of quizzes and a lot of recommended reading, so this isn't for those just looking for a way to walk off their hangover from the previous night. Like RS says about making shows for public television, this tour assumes an attention span.
Our experience of WWI and WWII sites is in process of transforming as the last people with direct connections to the events die. I was moved emotionally as much as intellectually by this Spanish Civil War tour by being able to stand in the footprints, as it were, of the actors involved in what went on there in Plaza Catalunya and surrounding it. In another generation or two this war will be compressed into the past, boxed and labeled and shelved like all else that happened In The Days Of Yore. I'm glad I got to get a taste of it before everything dries out.