What is permissible to read of English fiction, if at all. Is it an aveira to spend time reading fiction (if permissible) instead of learning?

The Shulchan Aruch (Orach Chayim 307,16) forbids reading romances and adventures that can contribute nothing to a person’s character and fear of G-d. That literature which does serve to develop and deepen a person’s character is not forbidden (Mishna Brura 307, 18). It is important to be selective and critical in choosing what to read and to be careful not to read material that could stimulate the yetzer hara [=evil inclination]. Rav Kook recognizes the positive power of literature and encouraged the development of good Hebrew literature as part of the redemption and teshuva processes. Do your reading at times when it is not at the expense of your Torah studies and therefore not “bitul torah”.