So, on my first character I found that the so called master trainers only trained too the skill of 90, so I wonder: Doesn't that kinda defeat the point of a a master-trainer in the first place?

Save the books till the end!

That. But it's hard

I'm waiting for a mod that lets me pick up books without reading them. I'll store them all until I'm maxed out, then spend an hour "reading".