Hello my dora ,long time to find you again in italki ,your welcome , First of all you can use both the last two answers but as the last one "Had I known etc." is emphatic the correct one in Toefl terms is "If I had known...". "If I had known you had wanted to read it. I would have returned it immediately." The real situation is that you did not know that I wanted to read it. This has to be followed by the past perfect. Now if you use "Had I known you had wanted to read it, I would have returned it immediately." you would be using an inversion which is used for emphasis. The 'if' is omitted. Both are correct but then the second one - the inversion - is slightly different in that it is more emphatic and more literary. This is type 3 of conditional sentences known as "Impossible condition". The verb in the if-clause is in the past perfect tense; the verb in the main clause is in the perfect conditional. If I had known of your arrival I should have met you (but I didn't know so I didn't meet you). Here we know that the condition cannot be fulfilled because the sentence refers only to past events.