It's about a guy named Thomas Hunter. Whenever he falls asleep in one "reality" he wakes up in another. One reality is the present day. The other reality seems to be some sort of alternate earth in the past (no guns, the people make their clothes/tents/etc.). Whatever Thomas learns in one reality he knows in the other (for example fighting techniques, etc.).
It's great fiction if you ask me but you have to have an imagination and enjoy fantasy b/c there are evil bat like creatures, some magic, etc..