Spellbook
The Spellbook screen is used to access (and where applicable cast) any of your spells.

This screen only shows spells that you have Learnt, and it does this by displaying a list of Magic Lines you know at the top of the screen.

When you click on one of the Magic Lines it then loads all the spells you know from that Magic Line, there are typically around 200 spells in most of the magic lines and this screen shows about 15 spells to a page.

When you click on a spell on this screen it will have the option to cast it, either on you or on another target if it is a spell that can be cast like this, otherwise it may have no option but to view further information on the spell.

Many spells can be cast only in combat, so may not let you cast them from this screen, this is fine and is not a bug.