If i may i recomend a book: it's Barbara W. Tuchman's "A distant mirror". It holds a lot of informations about the mercenaries in the 14th century Europe.
If you wanna know more about the mercs in the ancient world i'd recomend Xenophon's Annabasis; also, scarce information can be found in Thucydides' writings (classics.mit.edu/Thucydides/pelopwar.html- apparently i can't post the URL just yet) and even in the Illiad (but verry disimulated).
If you want to learn more about the ronin, i'd suggest that you look for books that focus on the warring states period. It's a period when many clans are defeated, resulting in a great number of masterless samurai