Allen's biography of B16
John Allen is the only thing making the National Catholic Reporter worth reading; his reports from Rome are generally very clean of biases, one way or the other, and he is probably one of the top American vaticanisti out there.
Allen is also well-known for his 2000 biography of Cardinal Ratzinger/Benedict XVI. It is good in many ways, but Allen's liberal biases do make greater appearances there than in his dispatches from Rome (as Allen himself acknowledged when it was pointed out; HT: David Jones).
But the biography also includes some outright errors (which are presumably innocent mistakes). The most notable is Allen's claim that Ratzinger was a student of Rahner's (he got this from Ralph Wiltgen's The Rhine Flows Into the Tiber). I first encountered this claim a year or two ago on a liberal Catholic's blog, and refuted it then (although Rahner was older than Ratzinger, he was never the latter's teacher). I found that the immediate source was Allen's bio, and I continue to see the claim made now and then.
Carl Olson links an '01 review of the book which points out some other factual errors Allen made, each with some theological implications.