Absolutely try to find a copy of David Parrott’s The Business of War. It’s also a massive book, and it’s more focused on the late medieval and early modern periods. BUT it does contain a very useful introduction that will give you an idea of the way mercenaries have been written about in European history. It also contains a little information in the first chapter that’s kinda high/late medieval. I imagine the bibliography and footnotes would give you some further reading as well. See what you think of the price and if it’s too hefty, try to find it at a library.

The best book on the topic (and the only sufficient treatment I'm aware of) is John France's Mercenaries and Paid Men: The Mercenary Identity in the Middle Ages. I'm afraid it's become dreadfully expensive, but perhaps you can acquire a copy through a local library.