Probably the hardest book to get 70+% for the Book of Engineering achievement.
First of course, the book gets randomly dropped during a battle around the beginning of story mode, probably after about 20 missions or so(which believe me is only the beginning of story mode)...easy 50 G's.
Now on to how you level it up. I'm doing this after I've already beat the game. What makes this book so hard to level up is that fact that you have no form of offensive attack...it's all defensive. So start with like a two to three star difficulty mission, then run up to some enemy horses and stick out your spears and let them run into you. Horses, elephant and camels are pretty much the only thing you can kill, so just rinse and repeat running up to them and squatting lol! As you level up, do missions with more stars.
Luckily the book of Long Spears is small (3 pages) and you get a lot of XP when you get kills with the long spears. The book should have pretty much been wrapped up into the Book of Spears but oh well. I did a level 3 mission (started with a level 1 squad) and killed 3 squads of horses and gained 10 levels.