I agree; a German friend of mine even wrote a song about that translation of 'toll': http://www.youtube.com/watch?v=GgvM4lpg5Lc
Though I tried 'Are the books awesome?' rather than 'Aren't the books awesome?' and I did it fully aware that 'awesome' was not one of the translations on hover, because I wanted to add it as a correct answer if it wasn't already. :)
You didn't have to use "cool" at all. You could have used "great" or "fantastic", but "good" is just not enthusiastic enough. The only reason you were shown "cool" is because the system always tries to show you an accepted translation that looks the most like the translation you entered. That may not always be the best translation, but it's a translation we deem acceptable.