Heichal Shlomo in the Rechavia neighborhood of Jerusalem used to be where the seat of the chief rabbis of Israel was. There was a rabbinical library there with 50,000 -60,000 seforim when it was the seat of the chief rabbinate. I have seen it listed in the Encycolpedia Jdaica (1st Edition) which was published in 1972. Does anyone know where the books and manuscripts went after Heichal Shlomo stopped being the seat of the chief rabbinate?
I found the following (my rough translation) here but I have no idea how authoritative it is.
The sale of the Hechal Shlomo Library.
There was a huge and excellent library, which from its inception served the Chief Rabbinate. The rabbinate was removed from the building and it was turned into a business. The library was reserved for the bulk of the Strashun Library of the Rash and his son R. Matityahu with rare and unexplored books and more. The library was almost completely eliminated without keeping any record of what was sold to whom. One can take comfort in the fact that not all of the Strashun Library was in Heichal Shlomo, and a section that is preserved in New York's YIVO Library is well preserved and is currently catalogued.