What are the themes of the novel Heartburn by Nora Ephron?

One of the story's themes you might like to consider is the impact of infidelity on relationships. The protagonist, Rachael Samstat, spends an inordinate amount of time trying to recover from the revelation that her husband, Mark, is in love with a woman by the name of Thelma Rice. This shocking news inevitably destroys the very foundations of Rachael's marriage and makes it difficult for her to move on with her life.
By way of exculpation, Mark tells Rachael that he hasn't been intimate with Thelma, but this only makes matters worse. Because if Mark, a highly-sexed individual, loves Thelma despite the absence of sex in their relationship, then that doesn't reflect very well on Rachael. For if there's one thing she's always been certain of, it's that she can give Mark all the love that he needs. However, Mark's revelations about Thelma indicate that this is no longer the case, and the realization devastates Rachael.