My personal favourites are the following listed below.
For info, (1) and (3) are good for kids below 10 years old and (2) and (4) for pre-teens and above. The authors' writing styles are reader friendly and they give a lot of examples and anecdotes to help illustrate what they are trying to say.
For the books written by Adele Faber, I felt as if she was talking to a friend when reading her books. I've also included the reviews on amazon.com for your reference.
1) Five Love Languages of Children
by Gary Chapman and Ross Campbell