I'm talking more about books that contained readable information. Sorry if I had expressed it in a poor way, but I mean books that, for example, were possible to find on libraries or shelves but for some reason were later removed. That's my standard pattern of "rare". I'm interested specially on what these removed books used to say, before became limited ones to own.
But of course, Tomes are pretty cool items and interesting to comment about, but I guess that they do not possess any information readable, as far as I know.