Translation:Raj's book is better than Julia's book.
Correct me if I'm wrong, but I think you could also say किताब से अच्छी है at the end, and the meaning would be the same.
From what I have read, they are not cognates. The English "better" stems from PIE bhād- ('good', 'better') while Persian "behtar" stems from PIE wesu- ('good'). However, I'm hardly any expert on PIE linguistics so don't take my word for it.
Well it's not actually nonsensical. It makes perfect sense and is understandable but isn't grammatically correct so sounds a bit silly.
That's precisely what apostrophe-s is a contraction for - 'Raj his book' becomes 'Raj's book'. It's archaic and not a construction that anyone would use nowadays.
in fact it is not, it comes from earlier English suffix -es, and is cognate with the genitive in other Germanic languages where it wouldn't actually sound like "Raj his".