What secondary sources do you have? Take a look at their bibliographies! A lot of academic medieval books even split off "Primary Sources" into their own section of the bibliography. It should be easy work to see which ones are available in modern English translation. ;) I don't have the book in front of me to check right now, but, if you're trying to get a holistic view of the conflict and its context, the bibliography in The Hundred Years' War: A People's History by David Green might serve you well.
For free sources available online, a great place to start is always Paul Halsall's Internet Medieval Sourcebook. It's got a section on the 100YW, even!
If it's the Hundred Years' War, you kinda gotta start with Jean Froissart, Chronicles. I'm pretty sure it's a contractual obligation. The Penguin Classics abridgement-translation is probably the best one for current purposes.
Hope that's a good start!