Is there something wrong here? Why is it just bok but not boken?
Some previous examples were like: Han är yngste sonen - He is the youngest son Han är den yngste sonen - He is the youngest son
As far as I understand, we do not always have to use "den" for superlatives in Swedish like we use "the" in English, but can we also use "bok" instead of "boken" all the time? Or, what is the exception here?
My personal observation is that whenever you attach a defining relative clause (i.e. a clause that is essential to pointing out what you refer to) to a definite noun, the suffix disappears. Here it is essentially "den sämsta bok som jag vet", just with the relative pronoun left out.
Right, but the den determiner refers to the comparison, not to the book. English actually works the same. You say "It's the worst book I know", not "It's the worst the book I know".
The det determiner at the start refers to the actual book, which is why you don't need a separate determiner later. You've already grounded the specific book through the very first word.
Still a little confused. I can't see how this case is different from the example, "Han är den yngste sonen" (from the post above by YavusGrgl). Here are a couple of sentences to maybe help figure out where my thinking is wrong:
"Det är den yngste katt" or "Det är den yngste katten"? Do you need to have the relative clause "Jag vet" to be able to use the indefinite form of "katt"?
Also, does this mean that "Det är den samsta bok" is also acceptable (without the "jag vet" relative clause)?
To expand on what devalanteriel said: With a superlative, it depends on whether it's attributive or predicative -- its position relative to the noun. Attributive means it comes before the noun (de sämsta böckerna, the worst books), predicative means it comes after a verb (de böckerna är sämst, these/those books are the worst).
There is one other point to consider. It seems for most adjectives the definite superlative ending is +e (e.g. varmaste, svåraste). For irregulars like sämst and längst, it is +a as shown above, but there is an optional +e ending which you might encounter and that is only used for the definite masculine singular (something with a true masculine gender, like an animal or human).
1) is the superlative's placement attributive or predicative, and 2) is it a regular or irregular adjective? I believe those are the two pieces of information to resolve your question :-)
I apologise, I read that clumsily. "I know of" is absolutely fine and I'll add it right away. In this case, there is zero difference in meaning between känner till and vet, and they're both accepted in the reverse sentence.
Generally, though, känner till means "know of" and vet just means "know".