Starting at the beginning:

Agatha Christie – an Autobiography (1950-65)
A hugely revealing narrative, even though it skips describing the eleven days that were the most famous of her long life. Agatha Christie always said that she never drew her characters from real life, but there are quite a few candidates that leap to the eye when reading this book.

Agatha Christie’s Secret Notebooks / Murder in the Making  by John Curran (2009-11)
An analysis of the creative process based on archive material found at Greenway. The genius of Agatha Christie lay in her ideas and these books comprehensively show how they were developed.

Come Tell Me How You Live  by Agatha Christie (1940-46)
A very enjoyable diary of the Mallowan’s trips to Syria and Iraq that gives insight into her resilience and what really mattered to her. Her true appreciation of other cultures comes across very strongly, and should give pause for thought to those people who consider her work to betray racist tendencies

A Talent to Deceive: An Appreciation of Agatha Christie  by Robert Barnard (1979)
The best, most balanced analysis of Agatha Christie’s work that I have so far come across. This is no hagiography, and that makes a refreshing change. The sort of writer whose opinions you will diagree with but still respect.

Agatha Christie on Screen  by Dr Mark Aldridge (2016)
An excellent and comprehensive history of all the adaptations of Christie works worldwide. The way things are going, it will need to be brought up to date on a regular basis.

Agatha Christie Cover Story  by Tom Adams and Julian Symons (1981)
Essential reading for anyone interested in the story and ideas behind the amazingly distinctive series of covers painted for Fontana by Tom Adams from 1963-73: the memory of these covers was the original inspiration for Swigatha.

The Agatha Christie Miscellany  by Cathy Cook (2013)
Exactly what it says, but fun to dip into and well-researched.

The Agatha Christie Who’s Who by Randall Toye (1980)
A listing of over 2,000 of the characters created by Agatha Christie. Randall Toye estimates that there are over 7,000 referenced in some way in the stories published in her lifetime, but that needed pruning, so the hundreds of butcher’s boys, kitchen maids, cooks, postmistresses and village parallels have been discarded. For obvious reasons, there is also no room for the characters in the tired short story re-hashes of the 1990s and since.

The following were also worth reading, if only to realise what an absolute global icon Agatha Christie has become:

A is for Arsenic by Kathryn Harkup (2015)
A qualified chemist and ‘science communicator’ considers the different poisons used in swigathas, with examples from 14 books, and then reveals their chemistry, action and history.

Who Killed Roger Ackroyd (Qui a tué Roger Ackroyd)?  by Pierre Bayard (1998)
The Professor of Literature at the University of Paris is convinced that Poirot’s solution to the murder of Ackroyd is delusional, and provides an alternative that best fits with the facts as described by the narrator. This books hints at the question: “If the narrator is allowed to hide information from readers in an effort to deceive them, how can we know which bits we can trust?”  The alternative solution propounded convinced me!

Agatha (2014)  by Anne Martinetti, Guillaume Lebeau and Alexandre Franc
A graphic biography, beautifully done, somewhat in the style of Tintin, with Poirot, looking like one of the Thompson twins, constantly turning up to plague the author. A very moving tribute.  

The Act of Roger Murgatroyd by Gilbert Adair (2006)
A spoof ‘homage’ to the Queen of Crime – a fun read, with a brilliant ending that would have had Professor Bayard falling off his chair. Since followed up by A Mysterious Affair of Style and And Then There Was No One, but one spoof is enough for me.

Other useful sources of reference and contacts:

Greenway House (Churston Ferrars, Devon)
One of Agatha Christie’s homes, now a National Trust property, almost a family museum, but well worth a visit for the grounds alone, and also the wider landscape.

The Torquay Riviera, the River Dart, Burgh Island and elsewhere appear in a few of her stories, and there is an annual Agatha Christie festival based in Torquay around the time of her birthday in September – a great time and place to meet fellow enthusiasts.

Online (Facebook)
There are some excellent (and some not-so-great) pages around, often attracting real experts on the subject. Surprisingly, Facebook fan pages have proved to be an invaluable source of interesting content and comment; the official Agatha Christie websites and Facebook pages are somewhat worthy and almost patronising.

Apart from the sensational @swigatha page, these are the some I have been following:

Agatha Christie Brazil
The Agatha Christie Fan Group
Greenway – the loveliest place on earth
The Hercule Poirot Fan Club!
Agatha Christie Appreciation Group
Agatha Christie Legacy

You do sometimes find that many of the thousands of followers on these pages are more interested in the TV and screen adaptations (and actors) than the books, but I have made some very useful and interesting contacts (and audience members for swigatha) on these pages.

Agatha Christie Legacy Magazine (Nick Smart and Raye Green)

As a result of a people coming together for a dinner at Greenway during the 2018 Festival, then re-engaging at Heathcliff House during the Festival in 2019, followed by the judicious use of facebook, enough contributions were solicited to create Agatha Christie: The Legacy, a magazine celebrating the centenary of The Mysterious Affair at Styles. It came out in March 2020, and it is hoped that it will be the first of a few! The first issue contained articles by Messrs Curran and Aldridge, indeed by swigatha too.