That's the topic of this post by Mark Tooley over at Juicy Ecumenism: George Washington's Birthday & Religious Library. Washington's religious views are subject to almost endless speculation, but the contents of his library and the outlines of at least part of his regular religious practice are fairly clear.
A lifelong Anglican (although in accounts of his later life it appears he refrained from communion), Washington's religious library was diverse but chronologically narrow. Aside from two copies of the Bible (one with the Apocrypha), he had no books, sermons or pamphlets dealing with early Christianity. Of the classical Protestant reformers, only Theodore Beza is represented.
What Washington's religious library did contain, as Tooley describes, is a broad selection of theological works from the 1700's, including sermons and pamphlets representing unitarianism, Methodism, Catholicism, as well as polemical works dealing with various topics. Books attacking deism and atheism are part of Washington's library, as are works on Christian apologetics and the Book of Revelation. Masonic sermons, explanations of the 39 Articles and the Athanasian Creed, and a defense of Quaker pacifism are present in the library as well. Most of the books, unsurprisingly, deal with Anglican or Presbyterian thought.
Interesting from a modern perspective is the large number of sermons contained in Washington's library, particularly in light of a religious practice of Washington's of which I was completely unaware: he regularly read sermons to his household on Sundays.
As Tooley puts it, "[t]he Washington religious library is eclectic." Given the broad reading and ambiguous sectarian views of the Father of Our Country, this should be no shock. Washington had friends from a variety of religious traditions, and it makes sense that books from a variety of religious traditions would find their way into his library. And while the contents of Washington's religious library may not shed direct light on Washington's private religious views, they do show some of his influences -- and provide a window into one aspect of his regular religious practice.
Related item: The Mount Vernon Ladies' Association of the Union, the charitable organization that owns and maintains Washington's historic home, has a page on its website exploring Washington's religious views. Well worth a read.