I was in elementary school in the 90’s, and I remember a book or series of books in the school library that I believe were all by the same author but may not have contained the same characters. I think both books had yellow hard-back covers. One of the books cover had a woman on it with a third eye, the eye was possibly triangle shaped. I’m not sure I remember the plot line of that book if there was more than one book. The plot-line I do remember though went as follows: A girl (and possibly her family) moves to a farm/rural setting. While out walking one day she runs into an invisible wall. Somehow she, and possibly a boy (or her family) get through the wall, and there is a hidden city behind it, possibly made out of crystal. The people in the city are either alien in nature, or are people with powers, and those powers and possibly telekinesis and/or telepathy. I think she has to figure out how to escape from this city. I’m hoping someone knows what these books are.