A spellbook will teaches you one of 4 ways to cast the spell, randomly chosen and deterministic per-spellbook:

• Verbal and somatic
• Verbal but not somatic
• Somatic but not verbal
• Neither verbal nor somatic (fully at will).

Various conditions prevent you from casting a spell if you are limited in how you can cast it: choking and being polymorphed into a form incapable of speech (etc) prevent you from casting a spell you only know how to cast verbally, and being held or stuck to a monster or having both hands welded to cursed gear (etc) prevents you from casting a spell you only know how to cast somatically.

You can read additional spellbooks to “upgrade” your casting ability in addition to refreshing your memory: e.g. if you can cast force bolt somatically but don’t need to speak, and you read a spellbook of force bolt that teaches how to cast it verbally but not somatically, you become able to cast it fully at will.

The motivation for this is to make large quantities of obtained spellbooks containing many duplicates of already known spells more interesting.

Not specified how the hero knows how to cast their starting spells; obvious options are:

• Always fully at-will
• Always requires verbal and somatic components
• Randomly chosen from among the four options