Once you have gained level one in any magic school you can continue to increase your knowledge in that particular school regardless of your role. Once you have attained a role that does not support a certain magic school you can only learn spells in that particular school if you have twice the level of the spell. I.e. in order to learn a sixth level stone spell as a ranger you would have to have level twelve stone magic. That means that you can never learn level seven spells in a non-supported magic school unless you have the enlightenment trait which again requires that you have been a zenmaster (from which role one rarely changes since it is eligible for learning all the spells). Also you cannot learn non-supported spells by invoking spellbooks. You can only learn them in connection with level-ups.
To find out which roles support which magic schools check Sazerac's table on
Page three of the Best Tips. Beware, though, that although he states that the warlock is only eligible for learning moon and fiend magic, the warlock can atually in addition to that learn sun and stone spells. Sazerac also states that the bard is eligible for learning fiend magic in addition to moon magic. In fact it is not so. The bard was at some point during the programming meant to learn fiend magic, but that option was taken off the programme again. Now all that remains is a small "bug" which causes the bard to learn level one fiend magic but does not make the bard eligible for learning the school. That means that the bard can only learn up to level six fiend spells as mentioned above.
