Would be pretty cool if reading a book had a very small chance of dropping a map or lead, depending on the location. I'm sure there would be other people looking for these antiques as well as us!
It would be a lot more believable to find a lead in an old stack of books you come across in an abandoned dwemer mine than say, from the corpse of a giant snake.
(unless some of the other trash loot included something like 'half-digested adventurer bones')
Would this idea only be for books you haven't yet read or also include the thousands of books you read prior to antiquities system was launched?
It would be at any book case, the books themselves aren't integral to the suggestion even though they open up upon use of a book case/stack.
Books rotate/refresh from book cases on a timer- there are some static books as well, but they don't change (some are needed for quests or exposition in an area). Each time you check a book case or stack could be a chance of that lead dropping for that cycle.
There is a lead that drops from reading something: the Pale Order Golden Band lead. People hated it because the note would vanish after someone read it, forcing other players to wait for it to reappear.
I answered the question the OP asked. If the question was "why not, " I would have noted that only a ESO developer could answer that question.
However, I was incorrect with my answer, just as OP was unaware that leads do drop from the treasure chest. Leads have dropped from treasure chests since the antiquities system was created.