Green Eggs and Ham is a great starter book, but IMHO the greatest of Dr Seuss's many great books is Oh, the Places You'll Go!. His mastery of "simple" English is stunning, kids find it enthralling, and the messages are surprisingly deep.

Here's the text, but the original illustrated version is so much better: https://www.poetrygrrrl.com/oh-the-places-youll-go-by-dr-seu...