Mystics and Thievery:

you can find the list of spells and thief ops in the official guide, well explained.
Here are some useful hints:

1. Mystics, most important spells:

Fertile lands - try to have it on 24 hours/7 days - it increases your food production which is very helpful - it helps you save space on farms!
Natures Blessing - protects from storms and droughts, you should have it on during war or when you have messed with a mage province
Fog - you should always have it always on during war, or when you expect to be retaled - it will slow down your attacker and give a better chance to hit back
Greater protection or Minor protection - it boosts your defense, so you should have it on 24/7 - but it's a must if you attack and you expect to be counter-attacked
Crystal Ball to gather information about your enemy
Crystal Eye - especially when looking for random targets, and to detect a dragon in development against your kingdom.
Inspire Army - reduces military wages and lets troops train faster - have it on 24/7!

2. Thievery

When you do a Spy on Military (SoM), Spy on Science (SoS) or Survey - one thief is enough to get a rough estimate - no need to risk sending hundreds of thieves.!

Be aware some operations are widley considered as black ops (see technical terms) and consider twice using them.

Use Rob Vaults, Rob Towers, Rob Granaries when you are short on supplies and don't need your stealth for war operations.

View "technical terms" of Utopia

Something about thieves:
It does cause some irritation with the newcomer, but thievery operations' success does not depend on the total number of thieves you or your thievery-target has, it depends on thief/land ratio.
If you have 4 thieves/acre, and your target has a ratio of 1 thief/acre, you have a huge chance of succes. For example: you have 2000 thieves on 500 acres of land, your opponent has 4000 thieves on 4000 acres of land - your thief ratio is better, so you will most probably succeed! It does not depend on the total number of thieves, just on the ratio! However, the bigger the difference in size, the harder it will be for you.
Of course, there are watchtowers ..... see below, and there are spells like Clear Sight .... as well as a random factor. So you can never be sure ;-)