Learn Polish: Czytam rosyjską książkę. - I am reading a Russian book.

Because it is the direct object of the verb czytam (I am reading). In Polish, direct objects usually take the accusative case. For feminine nouns ending in -a like książka, the ending changes to -ę in the accusative case.

Why do rosyjską and książkę have different endings if they are both in the same case?

Nouns and adjectives follow different patterns. While feminine nouns in the accusative case change their -a ending to -ę (like książkę), feminine adjectives change their -a ending to -ą (like rosyjską). They are both feminine and accusative, but their specific endings depend on whether they are a noun or an adjective.

How do I know this means "I am reading" right now, rather than "I read" in general?

You can usually tell from the context! Polish doesn't have a separate present continuous tense like English does. The single word czytam can mean either "I read" (habitually) or "I am reading" (right now). If you need to make it crystal clear that it is happening right now, you can add a word like teraz (now).

Is it wrong to say Ja czytam rosyjską książkę?