If your book is very thick, you may have notice that when you trimmed it, the innermost pages lost quite a bit of their margin. This is particularly noticeable if your outer margins are small.
So what can you do?
- Use lighter weight paper (it doesn’t bulk up as much and push the sheets out from the center)
- Enlarge your outer margins (it is much less noticeable)
- Adjust for creep in InDesign ‘Print Booklet’ window (set a negative number, which nudges all the content inward, to make up for the space cut off of the inner page).
While there are creep calculators like this, it might be easier to just do some experimenting to get it right, as you need specific details about your paper stock to calculate with the software.