Yeah 7.
The "Book of Achievment" is a leveling book, not a Skill Point book. If you re-rolled a character with 45 levels, then you get a book with 45 levels, regardless of his Intelligence or Skills. So if you take that level 45 book and use it on an Intelligence 7 guy, that new guy gets 45 levels, and Skill Points based his own stats, not based on the stats of the original character.
If you want to register a character that is optimized specifically for leveling fast and then re-rolling, that works. Many guys are doing this. The leveling build could have IN 6 so you can get Swift Learner, and pick the Loner Trait. These two things each give +10% leveling speed, so you can level a char 20% faster. Then re-roll him as a tank, for example, with IN 1 or 2.