Let me split this into parts:
Is it okay to show notability for a claim by citing a book rather than a web-site?
Yes! That's fine.
Please transcribe a quote from the book, if possible, so readers without access to the book can see for themselves what it says.
Please include an edition and page number, so people with access to the book can check that the transcription is accurate and in context.
Consider linking to the page in Google Books, if available, so people can check both even if they don't have access to the book.
Is it okay to show notability for a claim by citing a fictional source?
The rule of thumb is to ask whether the author is actually making the claim - whether they expect people reading the book to still believe it is true when they put the book down. If it is intended to be part of the fictional world, no.
Heads up! We have had Holocaust Denialist trolls in the past asking a lot of bad faith questions. As a result, while holocaust questions remain in-scope here, they tend to get a heightened level of scrutiny.