When I point a comparator away from the storage bookshelf, it does not emit any redstone signal. Might be more of a feature request than a bug- I don't know if you ever intended for it to do so, but it sure would make making hidden redstone contraptions easier.

Defo a feature request. I've never used comparator's.

Comparators are an insanely useful vanilla mechanism, I don't know why people ignore this thing....

Any specific way you'd like this to work?

Pretty much the way it works on chests and other vanilla inventories- if there's at least one item in the object a comparator should output a signal strength of at least 1, and if completely full emit a redstone signal strength of 15, with varying levels in between depending on the average of how filled up all the slots are. (unstackables have a larger fill weight than a single item of a stack of 64). The exact formula might be available for dispensers and droppers, since they appear to hold just as many items as the storage bookshelf.