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Я очень люблю интересную книгу - sounds clumsy, although it's grammatically correct.
But it would be better to say "интереснЫЕ книгИ".

Question #1 - you got it absolutely right.
Question no.2 -...

Ещё несколько лет назад они были очень популярны - Ещё - emphasizes that it was not that long ago when they was popular.
Всё еще популярен - "всё ещё" means "yet". But with a small feeling that its...