These I-SPY guides from Michelin are a great find even for toddlers. A good friend gave us the London Transport edition for E's birthday and he's been clutching it in his hands ever since. The miniature packaging is great for toddlers and they're lightweight and easy to carry.
The guides are intended to be interactive in the sense that kids collect points each time they spot something while they're out. I imagine that it's a great way to get older kids interested in their surroundings and I was pleasantly surprised to see what a big hit it is with toddlers interested in trains, boats and buses.
In an odd way it's an ideal children's book as it stimulates conversation with your child and encourages them to remember objects and details that they see during the day. E likes to flip through the pages pointing to the things he recognizes and it gives me a chance to ask him simple questions about our days out in the city. He's learned the word for clock thanks to the section on tube station time pieces and he now finds it very exciting that he has a wall clock in his bedroom (he never noticed it until now). My child may still be learning to master his speech and numbers but he can spot a tube station from a mile away! These little books are available at WHSmith and on Amazon.