It may refer to 'moral standing', but in this context it would be normal to drop 'moral' in English, leaving 'standing'.

Future books shall be of similar ... standing to (this one).

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Note added at 55 mins (2009-12-13 20:02:46 GMT)
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Re Asker's note below: You refer to the (ab)use of 'standing' in French, not English, where it tends to cover a wide range of attributes such as those you mention. Indeed, Belgians use the word in all sorts of circumstances where the English wouldn't dream of using it...

In English, as I already explained, we use it use it as a shortened version of 'moral standing', more or less equivalent to 'moral fibre' in some contexts.

I'm guessing they're using ethic in the etymological sense: ethos, "the characteristic spirit of a culture, era or community as manifested in its attitudes and aspirations" (Concise Oxford) -- so, as applied to a textbook, its approach.

I think it means the character of the work, with the implication of a similar high character.

From Funk & Wagnalls defintion#2 of ethical:

Conforming to right principles of conduct as generally accepted by a specific profession, a given set of ethics.

This is idea of a specific profession seems close to the French usage, and sounds to my ear like the English use of 'work ethic."

But to avoid the confusion with morals, I would suggest character from the Greek ethikos, ethos. As in "books of the same character." There is an implied high character, which is different than morals as do's and don'ts.