Looking for a fantasy book i read as a child

[Nerro]

Hi guys i am looking for the title for a book i read as a child. It was atleast 15 years ago i think but if i remember correctly it was a book from my mom so it could be much older. The book is about a boy who somehow travels into another world and i think searches for a way back. He later comes back into our world but travels into the other again a few times if i remember correctly. Every new time he goes to the other world he sees the time passed there a lot faster and there were years between his visits( time only passes faster in the other world while he is in our not other way round). While in the other world he gets hunter by a monster i think ( a kelpie/skelpie or sth in that dirrection not sure tho). Hope someone can help me i often get like a dejavu and have to think of this book and would like to read it again. Sry if anything is incomprehensible i am german and not that used to english

[Hayley]

Your English is excellent! 

I don't know which book it is, but it does sound vaguely familiar! I'd love to know if you find it!