Most people seem to think "no", and "no" is the safe choice, but opinions vary.
The typical arguments are: "The ads are not part of the author's intent, so you should remove them." vs. "They give a flavor of the original book, so you should keep them". This latter is particularly cogent when the ads are for other books by the same author.
Decide which of these statements best fits your own views in the case you're looking at; after that, it's up to you!