...Tschichold’s rejection of modernism...

He didn’t so much reject modernism as give up on the dogmatic side of modernism, the notion that modernism was superior, and that everything else needed to be pushed aside. This was a combination of his experiences with Nazi dogma and a pragmatic decision—if he wanted to survive by designing books sticking to modernism was pretty much out of the question. I think that people have a tendency to pick up on the mordant nature of the open letters Tschichold and Bill were writing and apply Tschichold’s feelings toward Bill to Modernism.

...less about typography and more about graphic design...

Don’t worry, they’re not mutually exclusive subjects, especially when Tschichold is being discussed.

Is my scope too narrow?

Not if you have access to a halfway decent library.

You need to get hold of the 'Reading Typography Papers' Volume 4 of 2000. The 1946 article ‘On Typography’ by Max Bill (“Typography is the design of of text-images, in the same way that modern, concrete painting is the design of rhythms of space.”), Tschichold’s rather condescending response ‘Belief and Reality’ ("Obeying good rules of composition and book design in the manner of traditional typography is not ‘putting the clock back’; but an eccentric style of setting is almost always debatable. Thus, the layout of Bill’s article is exciting because unusual, but is not to be taken as a model for general imitation.”) and Renner’s article ‘On Modern Typography’ 2 years later (“The Bauhaus style is, as Jugendstil and the arts & crafts of the Wiener Werkstätte once were, a station on the way to the style of our time. One should not linger too long at these stations if one does not want to miss the connection.”) are all reproduced with an introduction by Christopher Burke and Robin Kinross.

Speaking of Burke and Kinross, try to get Burke's book on Paul Renner, too, and Kinross's introduction to the English translation of Tschichold's The New Typography.

There is a new book on Tschichold by Christopher Burke: Active Literature. Given how great his Renner book is, I'm really looking forward to reading this one, as Tschichold is, I believe, a more pervasive influence. Between Burke's two books you should have a lot of information.