Today is the yarzheit of Rabbi Yitzchak Eizik Rabinowitz, most famous as the author of Doros Harishonim. He was also the founder of Agudas Yisroel.
Doros Harishonim is a history of the Jewish people which was written as a response to the historians (e.g. Graetz) who claimed that the Torah was not from Sinai, the Mishna was not from Rabbi Yehuda Hanassi, the Talmud was much later etc. The Doros Harishonim countered by making all the dates much earlier. He claimed that the basis of the Mishna is from the time of Hilllel and Shammai, and that the proto-Talmud was written by Rava and Abaye etc.He also claims that not only rules alone were given at Sinai but every possible range of opinion and every Halachic possibility were disclosed to Moses.
He had a tremendous wealth of knowledge, and brings a huge list of sources for everything. His history is basically the basis of modern ‘frum’ history (e.g. Artscroll). However, his sefer (actually it is 8 volumes in some editions) is out of print and almost completely unreadable. He writes with a clear agenda, and does not usually bring any outside historical sources to back up what he says. His ideas are controversial, and not really taken all that seriously by secular historians.
But it is a brilliant work of scholarship and worth having a look at if you can get your hands on it. There are some amazing ideas in the sefer that haven’t been popularised.

Here is what wikipedia says about him:

Yitzhak Isaac Halevy (Rabinowitz) (September 21, 1847–May 15, 1914) (Hebrew: יצחק אייזק הלוי) was a rabbi, Jewish historian, and founder of the Agudath Israel organization.

Isaac Halevy was born in Ivenec, Provice of Minsk, near Vilna into a rabbinical family, the grandson of Moredecai Eliezer Kovner. After his father was killed by soldiers, he was raised by his paternal grandfather. At 13, he entered the Volozhin yeshiva, where he was recognized as a talmudic prodigy. He held a number of communal positions in his early adulthood, including gabbai of the aforementioned Volzhin Yeshiva.

Halevy was influential in having R. Haim Soloveichik appointed to head the yeshiva, and he hosted the latter in his own house for months at a time.

May his Soul be Bound in the Bonds of Eternal Life