I think that the answer is that they need not be magical to function. A number of spell books will simply be the best book for the least money and will not detect as magic.
Some, perhaps the majority, have protective spells and\or glyphs etc cast upon them for protection.
Some, perhaps the majority, are careful about being found too easily and might have their magical auras concealed or hidden. An open spell book might detect as magical but a closed one might not - depending on its wards etc...
It would depend on the campaign.
Sigurd