As part of a series - why not - your credit idea is a great way to acknowledge the first designer.

This is terrible advice. I lurk these forums, but this comment (and the implications of the original post) are startling and no one else has spoken up. The original designer was paid by the publisher to do a layout for a specific edition of a book, not the book that you are being paid to layout. Only the rights for the content of the book are reverting back the author—not the rights for anything else (cover, interior layout, etc). Likely, the author never owned the rights for any of that. You are being paid to layout this ebook, not the previous designer. Is it ethical to copy someone else's work? No, of course not. If you had to ask you probably knew this instinctively. Is it ethical to copy someone else's work, get paid for it, and give a credit to the person you copied even though they haven't been consulted or compensated?

I don't think it's so clear-cut.

And giving due credit can be the saving grace.

Could you elaborate on why you don't think it's clear-cut? If someone is due credit aren't they are due compensation? The OP is asking about rote copying. This is not a conversation about homage, influence, reference, or revival (all very different conversations). The question posed was "Is it appropriate to copy someone's work?". Legally, no problem. No one will care. It's such a small project that even if someone did care it wouldn't be worth pursuing. But ethically? Do your own work. Have respect for the work your peers have done and don't copy it.

Everything is inspired by other things, and money isn't everything. To me giving credit goes a long way towards "compensation".

This isn't about being "inspired" by something—the OP is asking about the rote copying of an existing design. As I've already said: "this is not a conversation about homage, influence, reference, or revival (all very different conversations)". In my opinion, giving someone credit for something you stole from them is insulting. Would it be okay for me to make a rote copy one of your works as long as I gave you credit somewhere for compensation? Of course not. Can I copy a book design someone already did as long as I give them credit? Of course not. This is a needlessly reductive line of questioning, but I fail to see how you can make an argument for copying another designer's work. To belabor this point: the OP asked (in a post tagged "ethics") is it inappropriate to copy someone's work? He is not asking about being inspired by or taking influence or inspiration from. The OP did not read the text and come to similar conclusions as the other designer. The OP has only used the words "copy" and "replicate" not "inspiration", "influence", or any other word that might come with a modicum of ambiguity. The words we choose are important. It is not ethical to copy someone's work regardless of credit given.