You wouldn’t know this from their covers, but On the Human Being, International Photography, 1900-1950 and On the Human Being, International Photography, 1950-2000 are actually two pretty good books. I had wanted to mention these a while ago already, but I was unable to find them online - until this morning, when I was looking for something entirely different.
The respective histories of the clown and the baby in photography… But seriously, On the Human Being, International Photography, 1900-1950 and On the Human Being, International Photography, 1950-2000 provide a very useful overview of portraiture, covering the time periods 1900-1950 and 1950-2000 (you guessed that, didn’t you?). Both books contain three parts: A section with lots of photographs by many different artists, a selection of essays by various authors (some well known, others not so much), and a list of biographies of the artists.
Regardless on whether you like your photography with essays (that might contain intellectual gems such as this one: “It shifts between a reactionary impulse to restore the credibility of essentialist notions of subject formation, along with a revision of the traditional hierarchical and mimetic models of representation, and a conservative doubt concerning the radical, immediate and universal applicability of an open structure both in photographic and in socio-political terms.”) or not, the fact that these books contain so many different portraits by so many different artists makes them a wonderful overview of what portraiture can mean. And the portraits are chosen in a democratic way: Very well known practitioners are placed next to lesser well known ones.
As a simple survey of portraiture in the 20th Century, the two books are a very valuable source, and they are both to be had for pretty reasonable amounts of money, given that they are paperbacks, fairly large, and bilingual (English and Spanish).