Robert Wright's Excellent Website,
Line of Sight, is a great place to start.
http://www.wrighton.com.ar/
Robert lived here for years, and really studied the city. He has posts and photos about all kinds of barrios and buildings and physical attributes, and a great insight into the soul of the city.
(its not working right now- I hope that doesnt mean he is taking it offline, but even so, you can find it archived- like here, for example-
http://web.archive.org/web/20071028000416/http://www.wrighton.com.ar/ )
Also good is
http://baires.elsur.org/
another expat who writes in english about his love of BsAs.
There are a lot of great books about Buenos Aires that are pretty much only available in Buenos Aires.
If, like me, you are interested in the art, architecture, and design, the two Mimi Bohm books, Arte Noveau Buenos Aires, and Arte Deco y Racionalismo, are great.
Most of the buildings are still standing, and you can go see them in person.
http://www.mimibohm.com.ar/
For the real spirit of Buenos Aires, I suggest you buy this little book-
http://www.littlebookroom.com/authenticbuenosaires.html
and then go to as many of the places listed as you can.
You will learn what makes the city tick, and have some great, cheap meals as well.
I would agree, also that Time Out is better than Lonely Planet, and, once you are here, you can buy the Magazine format Time Out at most Kiosks, and its more up to date.
Here is an interesting book, free online- its from 99, so it is more historical than contemporary- a lot has changed in ten years.
http://books.google.com/books?id=0P...X&oi=book_result&ct=result&resnum=14#PPA34,M1