Elephant and Piggie (or anything by Mo Willems)
Shark in the Park (and other usborne phonics readers)
Slinky Malinky (from the Hairy MacClarey series)

For the younger one:
Going on a Bear Hunt
Postide pirates (and other barefoot books)

Mine are four and almost two, so I tend to read books that interest both. The Elephant and Piggie books are ODS's favorite books to read himself.