The Life And Death Of Planet Earth

What does “modestly successful” mean?

I am a student learning English, and I need some help with the expression “modestly successful”

Here’s a sentence from a passage about a book called “The Life and Death of Planet Earth”:
The authors wrote this book as a follow-up to their modestly successful tome Rare Earth.

What does “modestly successful” mean here?

what this means is that their first book was successful, BUT not highly successful. To be modestly successful is seen as a good thing – it is still success.