Learn Swedish: Ljudboken diskuterar vi ofta. - The audiobook, we discuss often.

This is called topicalization. By putting Ljudboken at the front, you emphasize that the audiobook is the main topic. Because Swedish uses the V2 rule (the verb must be the second piece of information), the verb diskuterar comes right after it, pushing the subject vi to the third position.

Is it wrong to say Ljudboken diskuterar vi om ofta?

Yes, that would be incorrect. Just like the English word discuss, the Swedish verb diskutera takes a direct object without a preposition. You don't use om (about) here, though you would use it if you changed the verb to something like pratar (to talk).

Why is the adverb ofta placed at the very end of the sentence?

In a Swedish main clause, a sentence adverb like ofta usually comes right after the first verb and the subject. Because the object Ljudboken took the first position and pushed everything else back, the structure becomes Object-Verb-Subject-Adverb: Ljudboken (1) diskuterar (2) vi (3) ofta (4).