PLYMOUTH ROCK AND THE PILGRIMS by Mark Twain (Harper & Row: $19.95). While this may not be the best of Mark Twain, that is praising with faint damnation: Even second-rate Twain is better than first-rate almost anyone else. The problem is that, unlike Twain's fiction, many of these 82 collected speeches and toasts--and even the art of platform speaking itself--have yellowed a bit with the passing of the times and passions that inspired them. Still, Twain fans and modern orators will find rewards. Who can improve upon the denouement to his sad tale of an unknowing child who aims a gun at Grandma and pulls the trigger: "He had supposed it was not loaded. And he was right, it wasn't. So there wasn't any harm done. It is the only case of that kind I ever heard of."