I first read this many years back, as the child the book was intended for. I liked it (partly because Tim has a dog that he fights to keep), but didn't fall in love with it. As an adult, my reaction was not much different. It's a pleasant, well written book, but not one that will really grip you. The concept of a big inheritance being problematic was new to me, but I think it's mainly the dog that I liked.
Toward the end, the book does deal nicely with the relation between Tim and his father - with Tim unaware of the extent of his father's injury. It also has some pointed but not too pushy lessons about good behaviour. The ending is quick and a bit slapdash, but overall the book is pleasant and harmless.