Ren Chartrand 
Spanish Guerrillas in the Peninsular War 1808 14 [PDF ebook] 

Constant Spanish guerrilla activity so drained the resources and diverted the attention of the French military that Wellington was able to advance against and overcome a numerically superior enemy. So many French soldiers were being used to counter the guerrillas and the threat that they posed that less than a third of the French army could be tasked with confronting Wellington. This book brings to life, for the first time, the formation, tactics and experiences of the Spanish guerrilla forces that fought Napoleon’s army. Using much previously unpublished material, it offers a vivid description of the guerrilla and his lifestyle.