This book-
http://www.amazon.com/Buenos-Aires-Cultural-History-Histories/dp/156656347X
Buenos Aires Cultural History- is not specifically about architecture, but is a great reference for walking the city- it discusses the history of various barrios and specific locations- where Borges lived, when, for example. Its very interesting to put things in perspective.
A couple more things to consider-
Ochavas- these are not unique to Buenos Aires- many Haussman era cities, like parts of Paris or beyond the diagonal in Barcelona, or even Washington DC, have some, but in BsAs, they were required by law for many years.
(an Ochava is when you cut the corner off an esquina, making an esquina not an esquina.)
Its turning a point into a line, and an intersection into a mini-plaza. Its a time honored historical trick of urban planners, and it humanizes a city. But it costs landowners money and land, so its rarely done in modern cities. But its one of the things that subtlely gives Buenos Aires character and charm.
Buenos Aires Photographer has some great Ochava documentation, and his site is great anyway.
http://www.buenosairesphotographer.com/
but there is another site dedicated SOLELY to Ochavas-
http://ochavasandba.blogspot.com/
The other key to understanding Buenos Aires from an architectural standpoint is Pulmones- which I have not found documented that much online. A Pulmone, literally, a "lung", is the again, legally required, open space in the middle of a quadra- the hidden courtyards in the middle of even the densest urban blocks. They are very Iberian/Muslim in their turning inward, and architecturally can be traced back thousands of years, but in the context of art noveau apartment buildings, they create secret, private spaces that are not visible from the street. I know my place would be nowhere near as nice without one.
Oh, and if you have not done it, or even if you have- there is no architecture walking tour in the world like walking Recoleta AND Chacarita cemeteries. Enough amazing design, stone, and metal work there to keep you learning for a lifetime. Chacarita is a hidden gem- its bigger, more peaceful, and less touristy and crowded than Recoleta, but has a lot of incredibly inspiring work in it. Plus, there is a cool feria on Saturday afternoons in the park outside, with everything from power ranger pajamas to incense to shoe repair.