Magic

As a thief, you'll never have the magical skills of a wizard, but just like they dabble in melee and covert, you can dabble in the arcane arts. Below is a list of the four commonly used spells, some you'll be able to cast, some will be only castable by higher level thieves or those with high Intelligence. There are more spells out there you can cast for fun, but that's all they'll be.

All spells use ma.po for gps use. JPCT and FFM uses misc for speed while CCC, TPA and EFF use defensive for speed. Ma.sp.special is required for more scrolls to be carried. If you carry too many you'll find they "escape" and lower your Int and harm you. With 200 ma.sp.sp bonus you can carry as many scrolls as you like, and only risk losing Int when handling them outside containers - speedy aliases to cast help! You also need ma.it.scroll to be able to read the scrolls themselves.

Chrenedict's Calcareous Covering

Skills required: Defensive, binding and earth.
Bonus required: Fairly high, about 200ish bonuses.

This spell gives you skin like that of a troll, as in it'll stop swords and daggers from damaging you. It's very useful spell, especially in CWC where the spell components and scrolls are located in one city. The ingredients for one casting of this spell are: One handful of white powder, one fish carved from varnish and one ball of congealed sap. Plus of course the spell scribed onto a scroll.

Endorphin's Floating Friend 

Skills required: Defensive, channeling, evoking and binding.
Bonus required: Low for channeling/evoking but requires more binding/defensive as you aim for larger shields

This spell will persuade a shield (of any type) to float around you and protect you from anything nasty. It will swoop in and block hits in much the same way as protect works. Most important is defensive, since that will determine how well the shield is attached to you. Make sure your binding is high enough. A trick to make a heavy shield float even if you are not up to it yet, is to make it lighter with GFR (a spell to lighten items). However, according to some, this makes the shield (temporarily?) less effective. The shield is effective against low NPCs, but is kicked out of orbit easily by higher NPCs (trolls!). It is possible to float several shields, but they knock each other out of orbit after a while. Unfortunately, it is often not clear when a shield is knocked out of orbit in combat, so it's a good idea to highlight the failure line in red or something. A good low level shield is a small shield, then progress to heavier types as your skills increase.

Sadly EFF has one new major drawback for covert users, having a shield floating around you is a bit of a giveaway of your position, to this end, you effectively lose a fair amount of hiding bonus when using EFF, I've not had a fiddle to find out how much yet though.

Floron's Fabulous Mirror

Skills required: Misc, channelling, divining and scrying.
Bonus required: Low for channelling and divining, about 150-200 bonus, scrying dependant on target.

This spell is the popular replacement for the crystal ball. Identify the container you keep your compact or mirror in as 'component pouch' and the spell will automatically get the component mirror for you.

Jogloran's Portal of Cheaper Travel

Skills required: Binding, dancing, enchanting, evoking and banishing.
Bonus required: Medium to cast unreliably (200). 50% reliable at 250 bonus, reliable at 300+

An extremely useful spell to have this is, I recommend getting it as soon as conveniently possible. This allows you to open a portal (gateway) to another location on the Disc. You can get a piece of rock "blorped" to the location you need to portal to, but this will vanish after using it once. Blorping is the method Wizards use to make a piece of jewellery remember its location. It's much easier to get some jewellery blorped for you and blorped jewellery does not disappear after one cast, although each cast will damage the blorp, so they need regular fixing. More valuable items damage less easily. You will need to contact a wizard to make these blorps, since thieves do not have the necessary octograve command.

Failing the spell can have annoying consequences. The door can appear as a burning door, opening and entering the door will then damage you. The most I suffered from this was about 1700hps, so be careful. Doors will also appear rumbling, which means they're unusable and will explode as you try to enter them. Also now you have mis-portals, this is where the portal will randomly open to another location, so make sure you look before you leap.

Skills required: Evoking, air, chanting, enchanting and channeling.
Bonus required: 250 bonus to cast for minor usage, 350-400 for good usage.

This spell creates a magical impact shield around you, which then absorbs any melee damage and some other type, such as Inhume attempts and other physical damage. A very handy spell to have if you have the high intelligence to get a decent bonus. Otherwise don't even bother with anything under 12-14 Int.

It uses two components in the form of a shield (small wooden is light) and ash which you can get from smithies with a broom.