I love the illustrations in Willy Ley's Exotic Zoology, which appear to be line art. Sadly my copy is a battered library-discard; either the front matter never attributed the artist or it was on a page that's now missing. Wikipedia and Goodreads say the artist was Olga Ley, but they don't offer any particular citation for the information and I can't find her name in searches of any of Wikipedia's reference links--so I don't know how much they suffer from ambiguous citogenesis.
Who illustrated Exotic Zoology and how do we know?