They changed the killer ??

I've just finished watching the episode The day the floods came.

Its an ok sort of enjoyable ep.

In the book Im sure the killer was Zacs father.

The tv show has the killer as Joanna Field

How on earth does a show writer have the nerve to change the original murderer. Its like theyre saying the author has no idea and they made a rubbish book.

This has completely turned me off the show. I understood they changed all the characters a fair bit to make them more watchable than readable.
But to change a killer?
wow