Perry pears are generally small, hard, dry fruit which are horrible to bite into - but yield surprisingly larger quantities of juice than you might imagine, and when fermented make the most sublime and interesting drink that is perry.
Some Perry Pear Varieties
More details can be found on the excellent website of the Gloucestershire Orchard Group
There is not a lot of accessible (or even inaccessible) written information on perry pears. However, the "classic" texts and illustrations are now available on the Vintage Fruit CD developed by Marcher Apple Network (a charity dedicated to reviving the traditional varieties of apples and pears from the Marcher counties). 107 perry pear variety descriptions are included (which does not mean descriptions of 107 varieties - some varieties have descriptions taken from several texts). The CD also includes 290 cider apple variety descriptions.