On one hand, it is good, but on the other -, how many of those books might interest you?

Perhaps this would sound better with a "really": "how many of those books might really interest you?".

But I let her off the hook.

This sounds OK, but to me, if you use "I let her off the hook" it almost sounds like you're some kind of authority figure, like the grocery store security guard or manager. As you're just a customer, I would use the idiom "I turned a blind eye".

5) My sister and her husband are still head over heels for each other.

This is correct, as it "with each other".

6) Keep me posted on how things will develop in the upcoming days.

I can't be with you, but I can cross my fingers.

This isn't really how we use "cross my fingers". You don't really say you will cross your fingers in the future for someone else. You can say "I can't be with you but fingers crossed everything will go well" or something along those lines.

UIt usually it happens at railway speedin the blink of an eye.

I'm not familiar with "at railway speed" but apparently it is used by some.

9) What you say here, on this page, is a bit all over the mapplace.

Again, "all over the map" is used but as a UK English speaker I've never heard it before.

I think, you should review it once again to getmake it more comprehensible.

10) It ruffles my feathers when my neighbor begins to drill walls.

This is OK, but when you "ruffle feathers", you cause upset or offence. I don't think drilling walls usually elicits such an extreme reaction! Perhaps I'd say here "It gets on my nerves".