Here is a shortlist of graded readers at about B1 for a girl of fourteen

 I was looking for contemporary stories related to women, love or music rather than classics like Pride and Prejudice.

I checked that they were all available on Come in. Everything in English! - Librería online de idiomas en Barcelona

I DO NOT get anything in exchange for adding links to these readers

Penguin Readers Level 5 B1: Boys Don't Cry - Penguin Readers

Penguin Readers Level 4 A2+: How to Give Up Plastic - Penguin Readers

 

Penguin Readers Level 4 A2+: The Kissing Booth - Penguin Readers

Penguin Readers Level 5 B1: The Pursuit of Love - Penguin Readers

Burlington Activity Readers 4^th ESO  A2+: BAR - Extraordinary Women 4º ESO - Burlington Books Spain

Burlington International Readers B1: BIR - Music Makers B1 - Burlington Books Spain

 

HOEPLI Level B1: The Beatles - Scott Wheeldon | Hoepli Editore

Oxford Bookworms Factfiles 3 B1 The USA – Oxford Graded Readers