General Information: Butter lettuce, also known as Boston or Bibb (though both Butter and Bibb may be slightly different varieties of Boston) is a loose-heading lettuce with large soft leaves and a fairly mild but persistent, slightly sweet, slightly astringent flavor that makes it a perfect salad base for highlights ranging from fruit (persimmon, (kiwi, pear,apple...), citrus, avocado) to fish (or hard-boiled egg), to nuts to sharp flavors like water cress or an upland cress like Shahi (which is also very nice just now, and a perfect substitute or its cousin watercress).
Butter lettuce's uses extend well beyond salads. The large soft outer leaves - sometimes comprising most of the head - can be used as the wrappers in 'lettuce wraps'- toritillaless tacos or whatever. Though there's probably a tendency to put cottage cheese or egg salad into such things, there's no reason to stop there.
Butter lettuce is also nice in regular sandwiches. You can put quite a bit of it into a sandwich which is still manageable, so that the lettuce becomes an important part of the filling, but one which doesn't overshadow whatever 'main' filling you use, even if it's fairly subtle flavored. As with salads, though, you should consider the lettuce's delicacy, and serve the sandwiches immediately after preparation.
Cooking with lettuce seems often to be overlooked. The large, soft leaves of Butter lettuce are very useful in soups, torn and added just before serving, either stirred into the pot or placed in the individual bowls with the soup added on top. The shredded leaves can also be added to stir fry just at the end of cooking.