And there generally is, most libraries have a catalogue or at least a directory explaining the numbering system at a topic level. Thing is, this sign doesn't just help people come to the library wanting help with these things. It helps people in the library who are experiencing this and didn't know they could find a book to help them.
Hell some people don't even know they need help with these things until someone offers.
Oh, hides as in 'secretly places them' rather than 'removes'.