Brill's new Pauly online, 1 April 2011 (Historiae, appeared between 105 and 109; originally in 12 or 14 books. Deal with the Flavian dynasty, from 69-96. Only books 1-4 and part of 5 survive)
Chilver, G.E.F. A historical commentary on Tacitus' Histories IV and V, 1985.
LC data base, 9-18-85 (hdg.: Tacitus, Cornelius. Historiae)