I'm wondering if tags for example Durret's Probability book would be allowed.
I think that when one is reading a book, it would be interesting what doubts other user had when reading the same book. It can improve our learning experience.
As already mentioned in the comments, there are a few similar past discussions1 - you can see that having a tag for specific book is in general not a good idea.
One of the problems is that we would end up having too many very specific tags. (There are only five slots available for tags.) And it is better to use tag to describe what the question is about, tag for source of the question would be a meta tag.
On the other hand, I fully understand that you would consider useful to see whether there were some other questions from the book you're currently studying. There were also a few discussions related to this and some feature requests suggesting how this goal could be achieved more easily.2
Probably the best thing to do with the tools we already have in our disposal is to encourage askers to include the source of the question. This is already recommended in the FAQ: How to ask a good question.
If askers include the information about the book, then you can use search to find questions you might be interested in. You can use Google - this has the advantage that it searches in comments, too. (Searching in comments is a bit more difficult.) Or you can use the built-in search. This only search posts, but you have the advantage that you can easily sort search results by votes or activity, filter them using tags or additional keywords, etc.