The most common route. Over 90% of research universities have an institutional license to Oxford Scholarship Online. Log into your library portal and search for "The Quantum Theory of Light." You can often download a chapter-by-chapter PDF for free.
Have you found a legitimate way to access Loudon’s PDF? Check the comments on academic forums like Physics Stack Exchange for updated library links—but always verify copyright. loudon quantum theory of light pdf link
Moving beyond Maxwell’s equations to treat light as an operator. The most common route