John Harris 
Harkaway’s Sixth Column [PDF ebook] 

An explosive action-packed war drama: four British soldiers are cut off behind enemy lines in British Somaliland and when they decide to utilise a secret arms dump in the Bur Yi hills and fight a rearguard action, an unlikely alliance is sought between two local warring tribes. What follows is an amazing mission led by the brilliant, elusive Harkaway, whose heart is stolen by a missionary when she becomes mixed up in the unorthodox band of warriors.