As a lifelong reader and bibliophile, I find the idea of talking about books I haven't read an anathema. That being said, Pierre Bayard has some very insightful things to say about reading in his book How To Talk About Books You Haven't Read. One important insight is that any act of reading a particular book is also an act of not reading other books. Like the economic idea of opportunity cost, you need to keep this in mind when you choose a book to read. Bayard also writes about how books are not just objective things, but are subjective. When you talk or think about a book, you are actually talking or thinking about your own mentally constructed version of the book. Others will have their own constructed version of the book which may have some overlap with your version of the book. A final insight is that a book has a place among all other books, a context in the collective library. When reading a book, you must consider this context.
As much as I find value in these insights, I did not find Bayard's book to be an enjoyable read. Perhaps is was too philosophical or obscure, but more likely I just don't like the idea of talking about books I have not read.