I'm currently reading Tacitus by Stephan Schmal (Studienbücher Antike, Band 14) and find it very helpful. So I googled Schmal and did find this Bryn Mawr Classical Review of the book. (There is also a review on Schmal's Sallust in the same series.) There is a rider in the review of the absence of certain epigraphic sources.
On a general note, I find the closing sentences of the review interesting:
… as Schmal points out, the difficulty of Tacitus is what makes him so good. Tacitean complexity is becoming more and more understandable in a world where nothing is as it seems, where wars are fought on television, and where nations and their citizens are constantly forced to question their identity.