A Brief History of Time is a popular science non-fiction book written by Stephen Hawking. It was written to make the esoteric subject of astrophysics (and theories about how the universe began) accessible to laypeople, with minimal technical jargon. The book begins with an introduction to physics concepts such as gravity and black holes, and concludes with analysis and search for a grand unifying theory (GUT).
This was the book that Aldo was reading when he is guarding the building that is holding Karl. ("Not in Portland") He appeared to be highlighting information from Chapter 7, 'Black Holes Ain't So Black.' It deals
with the nature of black holes and specifically describes the event horizon, which is the threshold of no-return. The version of the book he is reading is likely the tenth anniversary edition published in 1998, as the first top line of white text indicating the edition name is visible in the screencap, and that version has a gold cover.  
- Images and text from the two pages Aldo was examining from thetailsection.com  
- Note that these versions are of a slightly different formatting than the version Aldo is seen to be reading.
-  (See Chapter 7, p 2/8)