There are two broad types of magic in Habisfern - Rune Magic and Book Magic. The former is common in the sub-continent of Euressa where the campaign is set, whilst the latter is more common in the southern nations of the Old World. For this reason, Rune Magic is the one that is being detailed first, though both share a number of characteristics.
All magic in this setting is based around the idea of fixed spells that can be learnt and then used as often as necessary, possibly causing fatigue depending on your level of skill and the power of the spell being cast. A spell can be boosted to make it more powerful when cast - so a spell with a short range can be cast with a longer range if you have sufficient skill.
This allows some degree of flexibility, whilst keeping possible effects within a known set of spells.
Rune and Book magic allow boosting is slightly different ways. Because there are only a finite number of runes (they were taught to humanity by the gods) there is little chance of overlap between these spells, so a 'low level' rune can be boosted to be as effective as a 'high level' rune.
Book spells are invented by human wizards, and new ones are being invented all the time. Lower level spells are easier to cast, but don't scale as well as higher level spells. This is basically a simplified version of the magic system in my Myths RPG. A low level 'fire bolt' spell may have a range and damage that is hard to increase, whilst a high level 'fire bolt' might increase twice as quickly.
Sanity is an important part of both magic systems. Learning magic can be hard, but it can be made easier by taking shortcuts that can leave a wizard damaged. Risk of sanity loss is always a choice - if you don't want to take the risks then you don't have to. But the rewards can be very tempting.