Why does "Hominem unius libri timeo" use comparativus unius instead of positivus unum? Does it mean "I fear a man of one book (more)"? Or does that unius belong to hominem because it is accusativus+comparativus?
It looks like a comparative (cf. facilius, melius, and many others) but it is in fact a genitive. Thus unius libri is "of one book".
The word unus has an unusual declension:
- nom: unus, una, unum
- acc: unum, unam, unum
- gen: unius
- dat: uni
- abl: uno, una, uno
The same genitive in -ius is used by a couple of pronouns.
There is no comparative form of unus in Latin, nor do I know what "onner" should mean.