Here's why:
* Open compounds are two or more words that are joined together to form a single noun. They are usually written as separate words, but they function as a single unit.
* In this sentence, "reading room" refers to a specific place, a room designed for reading.
* The other words in the sentence are not open compounds: "bookcases" is a plural noun, "librarian" is a single noun, and the rest are function words (articles, prepositions, verbs).