As far as I understood, the nominalization is made with the neutral form of the verb followed by の or こと, as in the example
私は読むことが好きです。I like to read.
Though, I found this example where the verb is conjugated.
この本は、読んだことがありますか ? Have you ever read this book?
How is that possible?
この本は、読んだことがありますか ?
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".
こと is basically a lexical noun ‘thing/fact’, which is also used as a nominalizer. Like の, it is preceded and followed by the forms that precede and follow nouns.
In Japanese, no distinction can be made between noun modification and relative clauses
Japanese. A Comprehensive Grammar [2nd ed.]-Routledge (2013)
