Translation:The book is expensive because it is old.
When using want it's a main clause, so the standard word order is used, when using omdat it is reversed because it's a subclause. See points 4 and 5 here.
- Het boek is duur, want het is oud.
- Het boek is duur, omdat het oud is.
You can find more on word order in main and sub clauses here. (Take care it is an extensive explanation.)
Nope, it has nothing to do with causation. Coordinating conjunctions (nevenschikkende voegwoorden) link two main clauses. Dutch coordinating conjunctions are: en, maar, want, dus, of and doch (not a common one). All other ones (including of) are subordinating (onderschikkend), these link a main clause to a sub clause.
Of iscan be either a coordinating or a subordinating conjunction.