>If you have a translation, do you have the book?

No, you have a translation.

>Even if, like the Odyssey, there are hundreds of wildly varying translations?

Precisely why translations are not considered equivalent to the original text.

>What about an abridged copy? What about the Sparknotes version?

An abridged copy is not a copy of the unabridged version.

>If you have a copy of Pride And Prejudice And Zombies, do you have a copy of Pride And Prejudice?

No.

I'm honestly surprised these were the questions you chose to ask, when you could have asked what if you have 90% of the pages, or what if most of the pages are missing pieces because the book was shot with a shotgun, or what if the book was scanned and OCRed and all the "rn"s were replaced with "m"s and all the lower case Ls with ones. Hell, is a scan of the book close enough to having the book, or is it far enough that one can no longer be said to have the book anymore?