If your book is going to have magic in it, there needs to be some sort of consistency in order to make it more believable for the reader. It may not be necessary for you to explain every little detail within the actual story itself, but it could be useful to make a note at the start of your project, of a few key points.
Ideas you may want to consider:
- What – kind of magic is it? (Wand based/Elemental/Other)
- Who – practices it? (Everyone/a select few)
- How – is magic used? (defence/survival/how it affects your world)
- Limitations – that can weaken magic, or are unaffected? (Kryptonite/death/Other)
- Costs – of using/abusing magic? (extreme fatigue/insanity/death)
- Technique – used to wield magic? (spells & potions/wands/hand movements/telekinesis)
There are plenty of other factors that you may need to include for your specific project, and the internet is full of useful ideas that can help you with this. However, this short list could be a good starting point for those just beginning their journey into a new, magical world.