We booked a private tour through a travel agent. I was extremely happy with it. I don't remember the name of the tour company and the travel agent won't tell me. If you want the name of the travel agent I can tell you but I think it's shady they won't tell me the tour company (and gasp, allowing people to book on their own).
One of the best things about the private tour is that we didn't have to wait in the long lines when we got there. Our driver (associated with the tour) walked in and found our tour guide who had the tickets. We started at Auschwitz and then they drove us to Birkenau.
There were more signs at Auschwitz (although not a ton) but I don't remember any at Birkenau. I am sure you could download a walking tour though. I think Rick has a mini one in his guide book.
It did last long enough and she would have shown us more if we asked.
I saw a lot of other private tour groups and all of the tour guides seemed young, entusiastic, passionate about what they were doing. I found this to be true in Poland in general in people in the tourist industry (maybe I am biased because I am Polish) So you would probably have good luck no matter what tour you booked with. I definitely would not just sign up for a tour at the camp. There were at least 15-20 people and it didn't look like fun.
I usually can't pay attention during tours but I am happy I did this one.