so, legally speaking: if you’re not copying chunks of text from one to the other, and the books are visually distinct both inside and out: you’re probably fine
socially, the safest move would be to get approval from the original author
ethically… harder to answer, there’s a lot of ethics-of-art conversations that don’t have clear answers, or have different consensus answers depending on what community you’re working in. and then free work vs paid work goes and complicates things again.