Q. Someone that has the minhag not to learn Torah on nittel nacht, is he allowed to learn the halochos of nittel nacht written in seforim like Nitei Gavriel?
A. There is indeed in Nitei Gavriel and other seforim ample material regarding Nittel. (See also Vedorashto Vechokarto Y.D. 15, and questions 242, 960 and 1567 in this forum).
On question 241 we wrote; “On the prior question, (240) it was mentioned that Nitey Gavriel (End of the Chanukah volume) quotes eight different reasons for not learning Torah on Nit’l Night. One of the reasons (p. 245, #2) cited in the name of Korban Nessanel, is that it is due to mourning.

It is similar to the prohibition on learning Torah on Tisha Beav, as Torah learning effects happiness and those are occasions of grief, loss and anniversaries of persecution for us. It would stand to reason, that just like in Tisha Beav you may learn the laws that pertain to that day or about the sad experiences, stories and happenings related to the date, so it would during Nit”l.

Indeed the Zichronos L’ben Yerushalaim (R’ Y . Yelin – Tome II p. 19) relates that the Zais Raanon (The Gaon of Kutnah) would gather all his talmidim and use the occasion for exactly that purpose.”

Horav Shlomo Miller’s Shlit’a opinion is that even one who keeps this  minhag, is permitted to learn the halochos of Nittel on that night.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a