[Endoperez]

All magic paths increase magic resistance, but only for the spells of that path. Thus high Astral mage is more resistant to Astral spells, but not to Water spells requiring magic resistance check.

The magic-spesific boni are listed on page 27 of the manual. As quantum_mechani said, each point of Astral allows a mage to command 10 magical creatures instead of 5. Death and Blood also have leadership as the only bonus/boni, Death mages can lead 20 undead each but no magical units without other paths, and Blood mages are able to lead 5 magical and 5 undead units per level.