Poire William is a sweet pear liqueur named after the variety of pear the French call Williams Bon-Chretien, and which our secular society calls Bartlett. A Poire William can be either a true pear brandy, distilled from pears and made in Alsace in France and in Switzerland, or a hybrid, made by infusing crushed pear with a grape-based spirit. Stuart Walton, author of The New Guide to Spirits and Liqueurs, says that, like pears in general, Poire William can have a strong and luscious scent, while the taste is often "disappointingly mild."
In the absence of Poire William, pear nectar and brandy in some combination would probably be a fine substitute.
There is also an elusive preserve made of Bartlett pears and sugar, which is sometimes called Poire William preserves or jam, but that is almost certainly not what is called for in your recipe.