My kids love the Thomas series by Reverend W. Awdry. My son is really tough on books so the first set lasted several years through my daughter and partially through my son. These are so well loved in our house so I had to purchase them again.
The books are of small size and may not be that much fun for older kids but they are definitely a hit to my 2.5 years Thomas fan. He can't be apart with his little book case. He enjoys looking at the colorful book at bed time too. It's a good buy for him for $5.