There are some really historic material including some amazing artwork (some of which I believe is by some unfortunately little-known botanical artists like K. Cheluvayya Raju who died more than 70 years ago).
There is probably some interesting material related to G H Krumbiegel, Hugh Cleghorn and the Agri Horticultural Society.
There are also some interesting publications made by the Horticulture Department that are largely inaccessible - over the years I have had enquiries from the Royal Botanical Gardens Edinburgh for looking up a book called "Glass House – The Jewel of Lalbagh" published in 1991 that has hardly been seen by people who are interested in it. I have likewise had requests for archive data relating to John Cameron (Krumbiegel's predecessor).
There is also a great deal of as yet un-written history on plant introductions such as
https://en.wikipedia.org/wiki/Chayote ("Seemay Badnekayi" = foreign brinjal - one of the introductions to our diet thanks to the work at Lalbagh) as well as the numerous avenue trees (like the pink poui currently in bloom and chosen by Krumbiegel as part of his Ritu Samhara idea for Bangalore)
For a glimpse of the excellent artwork of an unfortunately little-known botanical artist - K. Cheluvayya Raju - see
and we have several other botanical artists who as yet have no articles on even on the en.wiki (for example the somewhat famous pair of Govindoo and Rungiah).
I have been postponing my own plans to research the archives there and would be very happy to join you in your meetings if possible. There are some active tree groups in India who will be interested too.
best wishes
Shyamal