Allow to forget all spell knowledge for a choosen memorized spell.
(or ID the book and prompt if you want to learn the spell)
Problem: unidentified books are often Scare Monster, Slow Monster, Know Alignment etc. which aren't of much utility for casters specializing in battle magic and will eventually clutter the spell selection screen mid-game, pushing the often used Magic Missile and Teleportation off the screen requiring additional keypressing or will change keybindings to often used bolt & ball spells, leading to mistyping due to muscle memory.
Current solution: ID the books before reading which isn't feasible early game, read the book only once to ID it and forcibly cast away the ~300 spell knowledge you usually gain with a Wizard after you build your PP pool big enough - this leads to less Le training, unfortunately. Marking spells works but it's a temporary solution.
Proposed solution #1: add a command which forgets the spell regardless of knowledge. You lose no PP for that and gain no Mana training for casting it 500 times to get rid of it.
Proposed solution #2: ID the book when attempting to read it for the first time and prompt the player if he wants to actually read it. The same prompt applies when the book is identified but PC has no spell knowledge of that spell to avoid mistyping mistakes when reading in bulk (like in the library for example).