Regarding the twenty-one cards and letters from Frege to Wittgenstein discovered in 1988 [None of the letters from Wittgenstein to Frege are thought to have survived the bombing of the Munster library in 1945], you can see into: Enzo De Pellegrin (editor), Interactive Wittgenstein: Essays in Memory of Georg Henrik von Wright (2011):
- Frege-Wittgenstein Correspondence, edited by Burton Dreben and Juliet Floyd, page 15-on,
and:
- Juliet Floyd, The Frege-Wittgenstein Correspondence: Interpretive
Themes, page 75-on:
they contain a record of Frege’s highly critical reactions to the Tractatus manuscript, which Wittgenstein had sent to him in December 1918 after having had the manuscript rejected by the literary publisher Jahoda and Siegel. And they
also contain his reaction to Wittgenstein’s frank criticisms (now lost, with
Wittgenstein’s side of the correspondence) of Frege’s later highly influential
philosophical essay “Der Gedanke”, an essay that Frege sent to Wittgenstein in an offprint.
See Frege's letter dated 28.VI.19 [page 51]:
You have certainly long awaited an answer from me and must want me to
comment on your treatise that you sent to me. [...] I have thus
been prevented from occupying myself more thoroughly with your treatise
and can therefore unfortunately give you no well-grounded judgment. I find
it difficult to understand. For the most part you put your sentences down one
beside the other without justification, or at least without sufficiently detailed justification. I thus often do not know whether I ought to agree, for their sense is not sufficiently clear to me. Surely the sense would become clearer with more detailed justification. In general colloquial language is too faltering tobe suited, just as it is, for difficult logical and epistemological tasks. It seems to me that elucidations are necessary to make the sense more precise.
