We read a book that certainly featured a unicorn. It had a mystical link between modern & ancient worlds that was made through either a church building or a door into a hill - quite possibly both; I know that at one stage someone's glove was caught in a rock, proving that the heroes had been through it. It culminated in four certain artefacts being collected in the same place - one of them was a sword - and the final act involved all the windows of the churchy-type building being shattered. I think the unicorn was called Findhorn but www searches haven't confirmed any of this - other than that Findhorn is a popular name for unicorns. I'll have to get hold of my fourth year teacher - he read it to us...