Reading spellbooks can make them shift place in your inventory
If the inventory contains more than one spellbook of a kind, they shift places if one of them is read.
Example: given the list of the following spellbooks in the inventory
spellbook of light (#1)
spellbook of light (#2)
spellbook of light (#3)
and reading the first one (#1) will result in the following list
spellbook of light (#2)
spellbook of light (#3)
spellbook of light (#1)