Guys, I'm not concerned at all with translating "wessen" and "whose." Here is the problem:

1)I feel drawing parallels in one's own language is helpful in understanding other languages.
2)I am writing an introduction to the Russian case system.
3)Since cases are largely unfamiliar to English-speakers, I wanted to use a few examples from English, including "he/him" and "who/whose."
4)I was then unsure if "whose" was really an example of case, since it acts much more like a possessive.
5)There are very few people around who know about the case system of older forms of English.
6)However, there are very many people around who know about the case system of German.
7)Since "whose" is equivalent to "wessen", I decided to ask about "wessen" in order to resolve my question about "whose."
So, why exactly is "wessen" classified as the genitive of "wer", when it is acting more like a possessive? For example:

You cannot modify "Buch" with "wem", "wer", or "wen."
But you can modify the meaning with "wessen Buch", much like "sein Buch" or "mein Buch."

Russian follows a much more predictable pattern; you cannot modify "книга" with either "кто", "кому", "кого", or "кем". "Кого книга" is not the same kind of construction as "моя книга." The "кто" does not become a possessive like "мой" or "свой". For "wessen", a different word is used: чей, which is in nominative form.

The end question is: Is "whose" really an example of case in English, as it is in German? And why exactly in German is "wessen"(the equivalent of "whose") classified as having a case?