This isn’t really a book about being 2d, rather creatures being flattened by a stars gravity thar they live in.
It’s called Dragon’s Egg And was a really tripped read.
The plot is essentially a group of scientists go study a neutron star with some advanced tech (related to what we have now) that helps them weather some gravity problems. They discover that the star is inhabited by creatures made from nuclei bound by the strong force. They try to communicate with these tiny sesame sized creatures, but 1 of their days = 0.2 seconds human time… so communication is tricky.