I love all of the books I've purchased from Ruqaya's Bookshelf, and appreciate this one very much. These books are wonderful for character development in our children, but also for parents! I find myself repeating lines from these books in every day life. I recently found myself saying, "____ needs it more than I do" (a beautiful line repeated throughout Green Dinosaur Umbrella) when gifting something to a neighbor.
Hajj has so many rituals but the story introduces and teaches some of these things without getting into too much detail (though even less would have been fine too, with more details on the back pages if needed, etc.). But perhaps it's a good middle ground, not too much detail so as not to lose the youngest readers and not too basic so there's enough for older readers.
I've been reading this book to my toddler and preschooler almost every day for the past year and it's helped me learn more about Hajj rituals and be able to visualize what it might be like (insha'Allah we will all be able to complete this pillar of Islam if we haven't yet)!