I was pretty sure I remembered this being stated in the item description, and sure enough it was:
製作技術を解説した軍用の教練書［製作時の修錬値獲得量が上昇（一定時間経過または一定量上昇で効果消滅）]
But then I checked out the English text online and it say this:
This comprehensive manual on crafting contains knowledge amassed over centuries by well-traveled Disciples of the Hand. It is known to inspire all who read it, granting a temporary boost to skill points earned from synthesis.
The "または一定量上昇で" bit seems to be completely missing from the English translation.