Headphones with less sound leakage also usually have more isolation too. Wearing headphones that isolate a lot when you're out and about could lead to getting flattened by a bus that you couldn't hear coming. Closed-back clip-ons, as long as you don't blast them, should be acceptable in a library. The sound emitted by them would be audible but not distracting to those around you. I agree about the comfort issue. I could only wear my KSC-75s for so long before the clips made my ears hurt. However, the EW9s have slightly different clips that may be more comfortable, I don't know. Also, the EW9s weigh 33 grams, as opposed to 43 grams, which may help. The foam pads didn't bother me, though, at least with the KSC-75s.
Something that I'm sure would work is a pair of earbuds like Apple's EarPods. Earbuds leak even less sound than clip-ons. For me they're very comfortable (but I did put a pair of earbud foams on them). Also, they allow you to hear what's going on around you. They're not the best in terms of sound quality, though. However, they only need to sound so good because I can still remember what the music is supposed to sound like.