John Flanagan 
Ranger’s Apprentice The Royal Ranger 7: Ambush at Sorato [EPUB ebook] 

Will and his apprentice, Maddie, are in Toscana to help with negotiations between the Toscans and the Arridi. When a scout brings word that their old enemies the Temujai are moving into Toscana, Will and Maddie leap into action, knowing that a Temujai advance is always bad news. To have any kind of chance, Will must convince another former foe, the Genovesans, to contribute crossbowmen to the fight — but the mercenary Genovesans require payment. A trap is set for the Temujai in the valley of Sorato, but can Will and his uneasy new allies make sure the Temujai fall for it?