As Carla Sulzbach writes in the introduction to her Masters thesis on R' Hoffmann (a translation of his work against Wellhausen):
Many descriptions of a famous person and his work nowadays may be entitled:
"[so-and-so]: the man and his work." followed by a derailed and integrated picture of the person in question. With regard to David Zvi Hoffmann. we immediately run into a problem concerning the biographical material. The authoritative biography still remains to be written. His correspondence. so far as it may still exist, would shed enormous light on his personality. his concerns and his scholarship: research into it also remains a
The contents of the articles that are available about Hoffmann are mostly of a hagiographical nature: they mention the extent of his scholarship. and then stop short and express regret that 'this is not the place to go into detail' concerning the particular aspect of his work that had just been mentioned. Furthermore. the authors of such articles state that there definitely is a need for more research on Hoffmann and his work.
However honorable the intentions of these writers and however well deserved the praise. the result is that we lack in depth studies of Hoffmann and his work. His very comprehensive writings. which cover a wide range of topics. such as Bible commentaries. works on post-biblical literature. halakhic responsa, historical studies. book reviews. political brochures about contemporary Jewish issues. etc. remain largely virgin territory. It is not the purpose of this thesis to attempt to answer the question of why Hoffmann and his work have been neglected. Nor is this an attempt to correct all aspects of this oversight. Future research in this area is therefore indicated.
Since the writing of these words, Prof. Marc Shapiro has printed some important letters in the periodical Ha'Mayaan ועוד ידו נטויה (there are also several letters in Parnes L' Doro) but other then that I don't believe much has been done to remedy the deficit.
The following articles will have to suffice for now:
1. Alexander Marx (R' Dovid Tzvi's son-in-law) - "Essays in Jewish Biography"
2. Louis Ginzberg - "Students, Scholars and Saints"
3. Aviad-Woltsbcrg, Ycshayahu - D. Z. Hoffmann in Guardians of our Heritage (translated into Hebrew in Sinai).
4. Leo Jung - "Path of a Pioneer" (his autobiography) has some interesting recollections on R' Hoffmann and other figures from the Hildesheimer seminary
5. Chaim Tchernowitz - Pirkei Chaim - a very nice description of R' Hoffman's lectures.
 I owe this reference to R' S. I am very much in his debt for directing me to it.