Oh well, didn't have time to reply yesterday; but then I thought if I wait a bit, I'll just have to second the obvious.

On my own shelf, and all well thumbed--

Sontag (both titles; look up the other one)
Szarkowski
Shore
Barthes
Rouillé
Jeffrey

--most were mentioned before.

For a masterpiece of a critical monograph, Looking in Robert Frank's The Americans. Kozloff's Theatre of the Face for a more encyclopaedic approach to one specific niche.

But if you really want to feel 20th century photography, nothing will beat getting and fondling, night after night, some of the monumental, seminal works of the period--Nachtwey's Inferno, Smith's Minamata, Salgado's Sahel, Koudelka's Gypsies...

Pretty much anything by Depardon, if you read French, combined with some rambling, incoherent thinking of your own, will give you great into the roots, the very heart of the published-back-then image: the mind of the photographer.

These just for starters...