The Wizard's Handbook
Here you will find miscellaneous information about spells which are somewhat queer, interesting or just stuff you should know about spells
All spells are disabled while Tenser's Transformation is active...This include your main character's innate abilities. Therefore cast all protective spells (Stoneskin etc) before casting it.
By drinking Potions of Genius, you'll let characters with lower intelligence (Like Aerie) get a temporary Intelligence boost. At this point you can scribe as many scroll as you want and when the Intelligence boost wears off, the scribed spells will stay in the character's Mage book. This is also helpful if you're playing above Normal rules and have problems with failing to scribe scrolls even with powerful mages. (Thanks Feenickz).
If your character is an Assassin/Mage...You can coat your Melf's Minute Meteors...Experment with this...Sol's Searing Orb for a Assassin/Cleric
Spell Immunity: Abjuration will work against Imprisonment...Usefull against certain Liches who like to cast Imprisonment a lot
Spell Immunity: Divination enables your invisible character to remain unseen even if Mages cast True Sight because all illusion and illusion detection spells are under the Divination school. (Thanks Dean J).
Mordenkainen Sword is resistant to physical magic damage...For example Magic Missile will do damage but Cloudkill would not
With the installation of Throne of Bhaal, all spells with level advantages (Like Flame Arrow) will be capped at level 20
The difference between level 1 Charm Person and level 3 Dire Charm is Charm Person will turn the character green and ready for command, doing nothing if not commanded while Dire Charm will turn the character green and make them attack the nearest enemy unless commanded to do something else
The Cleric/Druid spell Defensive Harmony lowers close party member's AC by 2, is cumluative and casts very quickly. Excellent spell and worth mentioning here.
Globes of Invulnerability (Minor and Full) are supposed to protect against Skull Trap, yet they don't. (Thanks Dean J). (A fix can be found at Baldurdash)
Blur (Makes AC better by 2 or 3)is cumluative so you can really lower your Mage's AC by using it multiple times. (Thanks Dean J).
Mirror Image can absorb damage from traps and certain spells...It will take one image for each trap. (Thanks Dean J).
A lot of people don't like touch spell but Vampiric Touch is a great spell to have. You don't have to roll to hit (Guaranteed hit), it gives your character 20-36 extra temporary HP and you can cast it on storage places like barrels and shelves (Cheating but worth mentioning).
A Mage can Vampric Touch him/herself and gain a free 6-36 HP temporarily without hurting him/herself.
Wizard Eye cannot be Imprisoned and is a good way to explore areas and not get you characters damaged. (Thanks Dean J).
Mislead will allow a Thief/Mage to Backstab again and again without becoming visible. (Thanks Dean J...Excellent Tip).
Cloudkill instantly kills Umber Hulks unless they save vs spell.
Imoen has one extra spell slot per level, kind of like a specialist Mage. (Thanks Luna).