There is question and differing opinion on every site on how to achieve this. The 4 books you have to 100% to be safe is sword, horse, bow, and spear. Just 70% the rest. There is a whole story line that goes along with this also which will be in the link provided. I am 100% positive you have to 100% the book of spears, since the book of spears is part of the story line to getting the book of engineering.
The best solution I found was posted by Botafogo007 on:http://www.xbox360achievements.org/forum/showthread.php?t=71...
Alright I just unlocked the achievement. Here is what you will have to do. You don't need to 100% horses like everyone says. You need to 100% Swords, Spears, and Bows. When you 100% a book, you will receive a silver weapon for that book. Once you get a silver sword, spear, and bow, the Avatar of War will appear and say "you are the one with the silver bow, silver sword, and silver spear..." He will then grant you the book of engineering. I've seen some who have said you have to beat one of every enemy type. I don't know if this is true, since I beat the game before going for this achievement.