I am translating a text into English about library guidelines concerning "protected" documents, i.e. books prior to 1850. I wonder if I can use the term "protected documents" for these kind of materials or if there is another, more appropriate term for them.
As mentioned by Hot Licks, "Rare Book Section" is current usage and the books you mention are, therefore, "rare books"
In the Rare Book Section of a library one may find rare and very old editions, and some or most of them may not be accessible to the general public.
The book was so rare, it was in the rare book section of a university ... - Ref 1
After World War I the Royal Library arranged a special room for its incunabula and 1635 rare books. This was the beginning of the Rare Book Section created in 1945 and consisting of... - Ref 2
“Yes, this is the rare book section and where many of the librarian's offices are located. We renovated this section first, since our collection is invaluable. - Ref. 3
You are looking for the formal terms from Library Science of Reserve or Reference. Reference is the most protected.
"Reference" is located next to the Reference and Research desk near the Information Commons computer lab. Reference books are for library use only and may not be checked out.
"DVC - Reserves" are textbooks and books used for current classes and have a more limited checkout period than regular books. These can be found at the Circulation and Reserves Desk opposite the entrance to the library. 3-hour reserve books are for library use only, and 1-day reserve books may be taken outside of the library.