One's magic book can get awfully busy if one buys all available spells and/or make your own. Can one remove spells that have become unwanted for one reason or another? I'm particularly interested in removing unwanted custom spells.
Deleting spells is not possible in vanilla game, however, there are mods and console commands to allow this.
First, here is a mod that will allow you to delete spells (custom spells included) with delete key.
If you do not wish to use mods, you need to use console tools.
To enable the console, make sure the bAllowConsole is set to 1 in your oblivion.ini file. The console can then be accessed in-game by toggling the 'tilde key' (the actual key can be ~, º, ¬, |,^, \, §, etc.. depending on your keyboard layout), found near the "1" key on most keyboards. The console prompt will appear in the lower left-hand corner of your screen. You can scroll the console output using the "Page Up" and "Page Down" keys. The console is not case sensitive: entering any of "tdt" or "TDT" or "TdT" will toggle the debugging text.
Note: Removing the spells as described below will remove them from your list of spells, but you can not create new spells with names similar to those you have removed (you will still get the error message A spell with this name already exists). If you create a spell and realize you made an error, you can not remove it and create a correct one with the same name.
For default ingame spells use command: player.removespell [Form ID]
For example, to remove the spell Calming Touch, which has Form ID 000A97CA, you must write this in the console: player.removespell 000A97CA
You will then get a confirmation message saying "Spell 'Calming Touch' removed from "
Note: You should not remove the default player healing spell (aka DefaultPlayerSpell, aka "Heal Minor Wounds") -- this spell is treated specially by the game as the fall-back spell after the player casts a scroll based spell. If you don't like the spell, then use TESCS (The Elder Scrolls Construction Set) to redefine it to do something innocuous -- e.g., turn it into a lesser power which raises your mercantile by one point for one second.
For full list of Spell IDs, see this page:
Removing custom spells works just the same way, but finding the Form ID is a bit trickier.
For versions of Oblivion not using the latest patch, try using this method:
The mod I use to to hide/delete spells is Clean Spellbook. It also allows you to manage your spells.
Side note: There seems to be a later version of the Clean Spellbook mod here (v2.0.3), compared to the one in the planetoblivion.de page I linked above. Putting this as a sidenote, as I have only tried the planetoblivion.de version (v1.3.0) and not v2.0.3.