H. Beam Piper's Paratime stories might be of interest to you. Try Lord Kalvan of Otherwhen. They are not, strictly speaking, time travel stories, as they involve travel to alternate histories, but they are good reading in any case, and may appeal to you if you like time travel stories. I would also suggest Larry Niven's Rainbow Mars for time travel humor. Some of Spider Robinson's "Callahan" stories involve time travel, usually in a very silly way. Leo Frankowski has a series starting with Cross-Time Engineer that you might try. Another Star Trek book to consider is Barbara Hambly's Ishmael. Orson Scott Card's Pastwatch addresses some interesting ethical issues with time travel (though I think he squeaks out of answering some of the questions he raises).