Why is 'Ich weiss ein Buch' not accepted?
Isn't this the accusative case? Shouldn't we say "Ich kenne eine Buch."?
This is the accusative case. Buch is neuter, so you use ein. It would be eine if the noun was female (e.g. "Ich kenne eine Frau" - I know a woman).
For other learners: You can find this explanation on the tips page for the Accusative lesson.
Would weiße be acceptable here?