While digital versions exist, this is a textbook you will reference throughout your entire career. The searchability of a PDF is great for quick lookups, but the hardcover third edition is a worthwhile investment for serious study, as the diagrams and typesetting are far easier to digest on paper during long study sessions.
The official Google Books page for the third edition offers a substantial preview (often 20-30% of the book). This is excellent for checking specific derivations (e.g., the quantization box volume) without paying. loudon quantum theory of light pdf link
Understanding how we actually "see" quantum light, including first-order and second-order coherence ( g(2)g raised to the open paren 2 close paren power functions). While digital versions exist, this is a textbook