McKay, H: Time of Green Magic

When Tom and Polly marry, their single-parent families find two halves don't make a whole. Max knows his dad will not be home again now. Abi finds herself a middle child and suddenly expected to share far too much. Especially with grubby little Louis. Then the family move home and stretch their finances to the limit renting an eerie, ivy-covered house. Strange things happen there when Esme, the art student, is the only one in charge.
Worried Louis summons comfort from outdoors, and a startling guest arrives. Abi reads alone, tumbling deep into books. Max loses his best friend and falls in love. All three grow further apart than ever. Meanwhile, Louis' secret visitor is becoming much too real. As understanding grows between the children at last, they too see the great spotted cat-thing that arrives in the night. Iffen is the problem they must find a way to solve. Where has he come from, and how can he go back? Will Louis ever manage to say goodbye?
A story of dark winter nights when Iffen prowled home and hearts, books became doorways that opened into worlds, and the ivy house was full of green magic.