OK my DS is 6 and reads very well. However, we've just had this home and, while it's not harmful or offensive (apart from being dated crap) I'm starting to fret about 'reading age' stuff. THe book's apparently aimed at 8-10 year olds, and it seems much too 'old' for DS ie it bores and confuses him because he isn't getting the concepts and the vocabulary's a bit fancy.
I mean, I'm not gagging to keep him to the Oxford Reading Tree (I am as sick as any Year 1 parent could be of Biff & Baff & Fucko and their no-internal-consistency stories), but now I'm wondering if I baby DS too much and should be trying to wean him off Thomas The Tank Engine.