the most helpful letters of recommendation are written like the best letters from tenure and promotion referees. That is, they’re experts in a subfield writing to peers who are experts in other history subfields, and so understand their charge to contextualize and explain the candidate’s research to an intelligent audience of non-specialists.That might not strike you as all that difficult an undertaking, but you would be surprised. One of the first things you learn when you are on a search committee is that you do not know the field, the players, or the historiographical issues in debate. Letters that can explain the importance of his work without spending three pages summarizing the dissertation, chapter by chapter, are extremely helpful. A good letter will also discuss the candidate's teaching experience and abilities. This topic is more important than you might think. In graduate school, we focus on interpretations and research, but most people will find gainful employment in the profession at schools where teaching is the primary focus. Letters about the candidates research agenda and contributions are good, but they only go so far.
Little also mentions something else that I have seen:
I’m sure you, the referee, are a big shot in your field, but not everyone will recognize your name and tremble that you have deigned to address words to us on the search committee.I mentioned much of what I am about to report in Blog LVII, so it is not new, and I am going to name names...as much as I can remember. I was on a search committee where a we had a candidate that we were keen on. He had a Ph.D. from Princeton, but was from Dallas, Texas and was teaching at a community college in the Dallas area. His letters from Princeton were basically useless. His dissertation advisor wrote a letter that positive, but no more than three sentences long. He basically figured that his name and the Princeton reputation would carry the day. (One of the problems with this approach is that we had two Princeton Ph.D.s apply for the job.) In the end, it was the letter from his supervisor at the Dallas area community college that really explained his strengths and moved him into the interview on campus list.
The comments from Little's post are also worth reading.
There is also a downside to writing letters of recommendation. Little discusses that in another posting: "Functioning Like A Senior Scholar with Junior Scholar Prestige and Pay."
Jonathan Wolff over at The Guardian is a bit skeptical about the value of letters of recommendation and hits on an important shortcoming that he makes clear in the title of his essay: "Academic Reference Inflation Has Set In, and Everyone is Simply Wonderful."
How do you get good letters? Well, if Wolff is right, then everyone gets them. That is a bit of an issue in grad school, since everyone knows how the game is played. But not always. See the essays by John Fea: "On Letters of Recommendation" and "More on Letters of Recommendation." Also see, Chris Blattman's article for Inside Higher Ed: "Will I Write a Letter for You?"