Philosophy is not my forte, and I hope readers will be able to give a better explanation of the content of Ketav Tamim. In short, it contains an attack on the philosophical, anti-anthropomorphic statements of Saadi'a Gaon, the Rambam and others.
The book and its author are mentioned in the contribution by the late Shraga Abramson to the Urbach memorial volume (Mehqerei Talmud 3). In his back-handed way, Abramson recounted his first encounter with the professor, at the first World Congress of Jewish Studies. Urbach gave a lecture about Ketav Tamim, and Abramson, who was a yeshiva student at the time, informed him that he had looked deeply into all extant sources about R Moshe Tachau. Urbach asked "And did you know about Ketav Tamim?" Abramson answered "Of course! How can you study R Moshe Tachau without knowing about Ketav Tamim?"
That's the story.