The oldest book in the Law Library is:
Nider, Johannes, 1380-1438. De contractibus mercatorum. [Cologne, Ulrich Zel, ca. 1468.  leaves, the last blank. 22.5 cm high. First edition. Catalog url: http://umil.iii.com/record=b1115735.
The oldest piece in the Law Library is a manuscript:
Calderinus, Joannes, d. 1365. De ecclesiastico interdicto: distinctiones decretalium (also a series of Questions disputed by Calderinus at Bologna between 1330 and 1346 and similar disputations of Paolo Liazari, canonist at Bologna).  leaves. 39 cm high. Catalog url: http://umil.iii.com/record=b1117332.