The Citadel written in 1937 is a fantastic book. And this is also autobiographical - Cronin himself was a doctor who came up through the hard times and then became a hugely successful writer. He later published his autobiography in 1952- "Adventures in Two Worlds"
The book still has a lot of relevance especially to Indian doctors where "ethics" is still a hazy area. Exploiting of patients with needless medication and un necessary surgery (one such which caused Manson’s transformation in the novel) is still prevalent in India.
This novel was once a English ancillary text book for higher secondary students in Kerala in the early 1980s. I would say this should be compulsory reading for all fist year medical students in India as part of their ethics course - do not know how many medical colleges have one ! In fact to quote "Wilkipedia"
“The Citadel is said to have contributed to the establishment of the National Health Service in Great Britain by exposing the injustice and incompetence of medical practice at the time”
Coming back to Cronin’s novels. there are quite a few common threads.
The Welsh mining town background is in quite a few like The Citadel and Hatter’s Castle. Then you have the "son/daughter of a Scottish protestant mother /Irish protestant father" theme like those in Green Years and The Ministrel Boy which again this is reminiscent of Cronin’s background . Also some cities like Winton, Glasgow and Dublin predominate in his novels. Ministrel Boy has some of the story set in Switzerland where Cronin spent his last years.
Cronin’s novels are very enjoyable to read, particulary if you understand the setting and the period (early 20th century) in which it was written – a time where the Protestant/Catholic divide ran deep - one of the reasons for the trouble in Northern Ireland later in the century.
His books are easily available in Amazon but in recent years seems to have disappeared from the shelves of most Indian booksellers .
Citadel especially is worth reading as is the Ministrel Boy and The Green Years. These are the best introductions to Cronin’s works before one graduates to tougher ones like Hatter’s Castle or the Stars Look Down..
I have read almost everyone of Cronin’s novels and have enjoyed them to the fullest.