My mother-in-law happened upon Gibblewort in one of those fab cut-price book shops where they have random books for £2 that would have cost around £40 from an, understandably now defunct, Mini IQ party. Both kids have had many books that they loved from that shop, and a fair few of their friends have received books from there as birthday presents.
Gibblewort the Goblin is published by Random House Australia, and is often quite Australian in its dialect. It follows the adventures of Gibblewort, an Irish goblin, who has various adventures, and is randomly insulted by many strangers, from what I have seen.
They are, without doubt, the longest books C has ever read on his own, which gives him a sense of satisfaction. Some of the language is challenging, but the print is very large, and there are plenty of pictures to break up the text. Most importantly, C finds it hilarious and reads it willingly. A great choice for a child beginning to read alone.