Ditquelle's Trail

This is where it all starts. The Lancastrians and the Yorkists are two warring factions of the Plantagenets, eventually united by the Tudors. You can download family trees here, which have been invaluable to me.

This is a perfect and concise definition of Salic Law and what it meant to those who held the thrones. Plugging a few sentences of this into Google may lead you to a more thorough article, or at least one that isn't all cut up like this one.

And then the Tudor Court came. The addition of Catherine of Aragon and the dearth of male heirs changed the way succession in England and all of Europe functioned forever.

Wikipedia is not always to be trusted, but this article is the only place all of the information is gathered so thoroughly. The part to pay attention to here is Isabella's power and role; the first queen regnant of England, remember, was her granddaughter, and Isabella had made sure in Catherine's prenuptial agreements that her children would have the throne of England.
It's interesting to note, by the way, that Catherine was known as "Catherine of Aragon" with no nod to Castile...this is because, ironically enough, she could not inherit the title under Salic Law.

A great, comprehensive (although overly flattering) resource for general knowledge about the Tudors.

More information than you will ever be able to read. Most of this is from primary sources, and is therefore reliable, but it is unedited and unorganized. If you know what you're looking for, this place is amazing.