In the first sentence 'IF' has been left out. This structre is very common in litrary books, I myself came across it while readin dubliners. * Had I not given up the college, I would have been an engineer now. Should you happen to pass a bakery, get some breads. The Inversion is also required that's why your third sentence is incorrect. the second one though sounds normal to me.

"Had I known that you wanted to read it, I would have returned it immediately." "If I knew that you wanted to read it, I would have returned it immediately." Both of these are correct. "I had known that you wanted to read it, I would have returned it immediately." This one doesn't work, because the first clause is an indicative statement, not a conditional. In the sentence it's clear that the speaker is trying to get accross that he in fact DIDN'T know that his interlocutor wanted to read it. Since it's something that didn't happen (but WOULD HAVE happened given different conditions), you need to add "if" (if that happened, then blah) or the inversion (had that happened, then blah). Hope that cleared things up a bit for you.

that is the third conditional, so you can say: 'Had I known that you wanted to read it, I would have returned it immediately', or 'If I had known that you wanted to read it, I would have returned it immediately'. ( 3rd conditional: if + past perfect, would have + past participle)