I've had this one in my head for some time and thought this should be discussed. Right now, any question that seeks advice on a 'product' is considered off topic:
Questions seeking product recommendations are off-topic as they become obsolete quickly. Instead, describe your situation and the specific problem you're trying to solve.
The rational behind this is is written down in a 2010 blog post aimed at Stackoverflow. The idea was that product recommendations go out of date quickly. While this is certainly true with some types of products (the example given deals with consumer electronics) it is far less true for other products, such as books.
Books tend to be a good source of nicely bundled information and they tend to have a longer useful lifespan than, to use the same example, consumer electronics.
Finding the good books, among the long list of books on any given subject, especially if you are not an expert on the topic, is hard. At the same time, most professionals can quite easily point out good books in their area of expertise.
(Some of the best books on information security I've read in the last couple of years were recommended by the various well known members of sec.SE; I'm pretty sure, I would not have found those books myself.)
So, the combination of the usefulness of receiving expert advice, the value of good information sources and the fact that they tend to have a long (compared to other products) lifespan, I feel books should be exempt from the "no product recommendations" rule.
I feel books are far too valuable a source of information to be excluded from asking recommendations on. I personally feel that if we want people to better understand information security and its various disciplines, we should encourage people to read good books on the subject.