I've recently started reading some books in the Oneworld Thinkers series. These are introductory works on important thinkers. I've read Ed Feser on Locke and Harold Noonan on Hume. The length is about 200 pages each, so they are a bit more detailed than Oxford Past Masters (now part of the "Concise Introduction" series) or Wadsworth's series, but still manageable.
I would be interested in knowing what books or series that others have found useful in studying intellectual history.