Portal fantasies (nice term) have always a been favorite type of story for me. I think it's the familiar character with the unfamiliar world that appeals, or something.

Barbara Hambly's Time of the Dark trilogy was one. I'm not sure what was wrong with it, but it never really jelled for me.

I loved Joy Chant's Red Moon and Black Mountain.

I think the first one I ever read was Andre Norton's Gray Magic aka Steel Magic. That one didn't hold up terribly well on rereading, but I loved it when I first read it at age eight or nine.