Dex doesn't always change the def, because it only applies a multiplier. So if you have 15 dex you get a X 1.5 in def.

If you get to 16 in dex from 15 then your def may not change (dor instance if you have 2 def and 15 dex you get 3 def overall. With 16 dex you still get 3 def overall because the game rounds up)

I'm not sure how to find the info card.  I'm looking at the pop-up text for DEF which shows how DEF is calculated.  This text states the dexterity that is used in the calculation, and it doesn't change after using the book.  I see that same thing for a strength book--the pop-up text which shows the calculation for attack value states the strength used and the strength does not change after using a strength book.

when your mouse is over the ovjet, an info-card is displayed on the right of the screen. You should look at the bottom of that infocard.