My comment was not really regarding how to change minor ticks
but about where to put them on logaritmic axis.

On figure 3.4 you have added labels to some of the minor-ticks,
but I still think the subject could do with a short discussion when
you are providing a nice introduction to logaritmic plots.

I discussed this sometime ago on the gnuplot mailing list

I just looked over chapter 3.4 on logarithmic plots, and like it allot.

However, I would suggest that you include a discussion in minor-ticks on log scales.

I spent some time figuring out what is best practice, and my conclusion was that this is a very hard question with no clear answer specially if one can not fit 9 minor tick marks.

Furthermore, at least 4.xx version of gnuplot have rather strange defaults on this matter (maybe version 5 is better, I did not find time to investigate this).