Three ways to get the Secret Book of Giants for enchanting skills: the third profession quest, the castle blacksmith and the Olympiad, how to agree with a future hero, and where to look for the NPC that enchants the skills of your class.
Many look for information on how to get the books of giants, 
Secret Book of Giants; I will say at once that for an ordinary player these books are rather hard to get.
- One book you can get on handing in the third profession quest, but that is a long and tedious business.
- You can craft these books at the castle blacksmith (if you are a castle owner, of course) with the crops handed in by players as manor and ripened in the castle; the grander the castle, the more you can craft. When I had Dion, I did not even try to craft these books: there is no point, since in a low castle very few come out. But when I was the owner of Goddard, I crafted so many skill enchant books that I enchanted myself, enchanted all the skills on my supports, stuffed the whole clan warehouse and gave a heap of books away to friends.
- And the third (and last) way to get the books of giants is to become a hero at the Olympiad. Having exchanged at the Olympiad manager the points you earned for Noblesse Gate Passes, you can exchange the passes at the same manager for all sorts of goodies like blessed enchant scrolls, accessories and... skill enchant books, and there will be very many books (I also gave them away to friends all the time).
Yes, you will say, heroes are few and becoming one is hard, so what is to be done? If you are a noblesse, you can come to an agreement with a future hero: you feed them your points at the Olympiad, and when they take hero, they give you books of giants. How many to ask for? Considering that one Secret Book of Giants costs only 5,500 Noblesse Gate Passes, you should agree on no fewer than 10 books. And one more thing: if you did manage to gather 50 or more points at the Olympiad but did not become a hero, try exchanging them for passes at the beginning of the month; there may be a surprise.
The full description of the Olympiad and of which items can be got for the passes: Grand Olympiad and Hero status: rules, points and rewards. And the books can always be bought on the market, especially at the beginning of the month, though the prices for them are sometimes not too adequate.
And yes, everybody always wants to know where to enchant skills for a particular class with this book of giants. The general rule: look for the NPC with the skill enchanting function for your class where you took the third profession quest. For example, if we took the third profession quest for a crafter in Rune from the head blacksmith, we should poke at the NPCs standing nearby, and one of them will have this function. The enchanting itself is described in the article Skill enchanting: requirements, routes and what happens on failure.
After the Interlude chronicle this book was renamed and came to be called Giant's Codex, but its action remained as it was, that is, ordinary skill enchanting; other books of giants were also added later: for safe enchanting, for changing the direction of the enchant and for cancelling it.