It only increases the chances to get a crafting bonus, but has no effect on the amount of the bonus (that is, it will not help you get 25% bonuses).
Since the chances to get a bonus (at least 1%) are pretty high, it does not make much sense to use them. However, sometimes you will find that in a weapon with several stats that can gain a crafting bonus, some of them don't get one. Here's an example:
See how the damage didn't get a crafting bonus there? A Quality Crafts or Gunsmithing Monthly could have prevented that, and that bonus would have been anywhere from 1% to 25% (of course, the amount of the bonus still unaffected by the book). It's pretty rare that a stat doesn't get at least a 1% bonus, so it's not really worth using those books unless you are crafting a very rare weapon and you prefer to spend fuel to try to get the very best item possible.
Edited by Lana Woo, 30 May 2017 - 06:47 PM.