You're right that that is how nominalisation works. Whether you would call it nominalisation in this example is, I think, debatable.
...ことがある is a set phrase which means "there are times when..." / "there is a time when". So when you convert it to the question form you get "are there times when... " / "is there a time when ... " which more naturally becomes "have you ever ...".
I say it's a set phrase but you can analyse it quite simply. こと is a word with many meanings. If we pick 'occasion' as the translation in this case then we get the relative clause 読んだこと meaning "occasion when you read". I'm sure you already know that Xがありますか means "is there X", so you can quie naturally understand the whole sentence as "is there in occasion when you read this book", i.e. "have you ever read this book".