Biblia Sacra, a Bible printed in Venice in 1476.  An indication of the care with which Thomas had studied this precious volume is his account in the History of Printing in America. Impressed by the quality of the typography—an art still in its infancy when this bible was printed—he described what he observed “in technical language, no pick, blot, blur, friar or monk is to be seen in the work.” In other words no letters filled with ink, uninked letters, or black blotches marred the pages. As a good bibliographer, he described the collation, concluding that a large font of types would have been necessary.  He also singled out the “handsome” illuminated ornaments, particularly this red and blue opening of Genesis. 

Thomas’s notes on a loose gathering of paper laid into the volume gave its provenance and a report on work he did to preserve it. “This book was formerly the property of Dr. Increase Mather, for some years president of Harvard College.  At his decease it went to his son Dr. Cotton Mather, and when he died it came into the possession of the Rev. Samuel Mather. It belonged to his daughter, Mrs. Hannah Mather Crocker, who presented it to me.  Isaiah Thomas. This book was not rebound, but the waste leaves at the beginning and end, and a leather cover over the old boards, were put to it in 1808.” [read page here]

Thomas’s acquisition of this volume for study and inclusion in his own History of Printing in America, and his knowledge that Crocker was giving away other material from the family collection may have prompted him to arrange for the purchase of the remaining volumes.

The Amerian Antiquarian Society, 1812-2012 - A View at the Bicentennial
The Lengthened Shadow of a Man - Isaiah Thomas and the Founding of the American Antiquarian Society, 1812-1831