I think one can learn a great deal by reading the letters to and from his mother, now that we have three volumes to access--and also references to her that can be found in the indices. But that is one kind of source that may not explain, for example, things he kept from her. For example, he was worried that she would see some of the really crude poetry--I think I remember this comes from letters with Aiken. But it's a start.
>>> "Materer, Timothy J." <[log in to unmask]
> 12/12/12 11:05 AM >>>
> Eliot published his mother's play, Savonarola. As I recall his intro or forward was quite revealing. Try Howarth's Some figures....
> P. M.
My recollection is that there is nothing personal about Mom in the preface and is instead a dry discussion of the form of the historical verse play.
As for Howarth, there is a lot of information about his mother's contributions to the St. Louis community and some on her poetry. But it is outdated.