Book collecting is bringing together of books which in their contents, their form or the history of the individual copy possess some element of permanent interest, and either actually or prospectively are unusual, in the sense of being difficult to acquire. This condition of rarity, which figures much too largely in the popular view of book-collecting, is entirely subordinate to that of interest, for the rarity of a book devoid of interest is a matter of no concern.
On the other hand, so long as a book (or anything else) is and appears likely to go on to be easily procurable at any moment, no one has any reason for collecting it. The expectation that it will always be easily procurable is often groundless; but so long as the eagerness exists it restrains collecting, with the result that Horn-books are much rarer than First Folio Shakespeare