Books where someone in a relationship starts liking someone else

[Mallory]

Hey! I’m looking for a book where someone who’s in a relationship starts liking someone else OR a book about someone falling for someone who is in a relationship thx!

[Shawna]

Girlfriend Material - someone in a relationship starts liking someone else

Miss Match - someone falling for someone who is in a relationship

[Eric]

E M Foster's A Room with a View is a good oldy .

[Pony]

I haven't actually read this myself (I couldn't personally get into it), but Something Borrowed.