My 26mo daughter loved this book while potty training and still requests her 'Princess Potty book'.
The story that Princes Polly thinks it's OK for baby sister to have nappies but she wants to be more grown up, like the rest of her family, is something kids could easily relate to. The pictures are really cute but clear so don't mystify what is going on in the potty! We especially liked the 'Cheer' button, useful on all sorts of potty-training related occasions :) There are pages of potties and pants to help Polly choose from which is fun like shopping.
As well as the basic training messages which are nicely reinforced e.g. keep on trying, practice pulling her pants up and down (don't laugh it's an essential skill :) the book covers that it's fine to have accidents and she is still trying very hard and a clever girl; and also - and I think more unusually - (and sorry about this) that your little one might find having a poo in the potty much more difficult to adjust to when she has been used to using nappies but that's ok. This might really help some kids. The book ends on a high note that Polly will now be able to teach baby sis how to use the potty when the time comes.
Overall nicely thought out and beautifully illustrated. Recommended.