This answer to a frequently asked question has been prepared by Ruth Allen.
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Because the Abbey books were not published in 'reading order', reading the series according to the publishing dates, while interesting, does not give the story in chronological order according to 'Abbey Time'. The booklet, The Books of Elsie J Oxenham (available from the EJO Society at £1.00 plus SAE to members, £1.50 plus SAE to non members) shows the three 'sets' which make up the Abbey Series, listed in publication order, but numbered A01 (Girls of the Hamlet Club) to A38 (Two Queens at the Abbey). The booklet also contains cross-references to connecting series (Camp Keema; Kentisbury, etc.) which are listed separately on its later pages. Here I have set out the 'pure' Abbey Books themselves, with a few notes. At the end I discuss one or two titles that are sometimes included in the main series, and give my reasons for leaving them out. [see also FAQ 8: What are Abbey Connectors?]
First Generation Abbey Set:
A01 Girls of the Hamlet Club 1914 Chambers
A02 The Abbey Girls 1920 Collins
A03 The Girls of the Abbey School 1921 Collins
A11 The Abbey Girls Go Back to School 1922 Collins
A13 The New Abbey Girls 1923 Collins
A14 The Abbey Girls Again 1924 Collins
A15 The Abbey Girls in Town 1925 Collins
A16 Queen of the Abbey Girls 1926 Collins
A12 Jen of the Abbey School [see note 1] 1927 Collins
A17 The Abbey Girls Win Through 1928 Collins
A18 The Abbey Girls at Home 1929 Collins
A19 The Abbey Girls Play Up 1930 Collins
A20 The Abbey Girls on Trial 1931 Collins
A21 Biddy's Secret 1932 Chambers
A22 Rosamund's Victory 1933 Harrap
A23 Maidlin to the Rescue 1934 Chambers
A24 Joy's New Adventure 1935 Chambers
A25 Rosamund's Tuckshop 1937 R.T.S./G.O.P.
A26 Maidlin Bears the Torch 1937 R.T.S./G.O.P.
A27 Rosamund's Castle 1938 R.T.S./G.O.P.
Retrospective Series: [see note 2]
A04 Schooldays at the Abbey 1938 Collins
A05 Secrets of the Abbey 1939 Collins
A06 Stowaways in the Abbey 1940 Collins
A07 Schoolgirl Jen at the Abbey 1950 Collins
A08 Strangers at the Abbey 1951 Collins
A09 Selma at the Abbey 1952 Collins
A10 Tomboys at the Abbey 1957 Collins
Second Generation Abbey Set:
A29 Jandy Mac Comes Back 1941 Collins
A28 Maid of the Abbey [see note 3] 1943 Collins
A30 Two Joans at the Abbey 1945 Collins
A31 An Abbey Champion 1946 Muller
A32 Robins in the Abbey 1947 Collins
A33 A Fiddler for the Abbey 1948 Muller
A34 Guardians of the Abbey [see note 4] 1950 Muller
A35 Rachel in the Abbey 1951 Muller
A36 A Dancer from the Abbey 1953 Collins
A37 The Song of the Abbey [see note 5] 1954 Collins
A38 Two Queens at the Abbey 1959 Collins
Damaris Dances is often included in the main series - partly because it is relatively easy to come by, and is very closely tied in, with Rachel and Damaris taking a major part in the series from Guardians onwards. When reading it as part of the Abbey Series, after finishing Maidlin to the Rescue, read pages 1-172 of Damaris Dances, then read Joy's New Adventure, Rosamund's Tuckshop and Maidlin Bears the Torch. Then resume Damaris Dances from page 173 to the end. Page 173 of Damaris Dances gives a very brief résumé of what happens in the three books that take place during 'the [autumn,] winter and spring of hard work in Milan'.
Adventure for Two and Damaris Dances overlap quite a bit, and the series they lead into finishes with Margery Meets the Roses, which itself, by introducing the Kane sisters, leads back into the Abbey Series with the first mention of them in Robins, and their introduction into the series, Nanta in particular, from Fiddler onwards.
Other closely connected books are the Kentisbury set (only Patch and a Pawn and The Secrets of Vairy are not in the main Abbey Series) and the Quellyn/Woodend set (only The Girl Who Wouldn't Make Friends and New Girls at Woodend are not in the main series). The background leading up to Abbey Girls Play Up is told over three books: The Crisis in Camp Keema, Peggy and the Brotherhood and The Camp Mystery. Jen of the Abbey School, itself technically a 'retrospective' in that it was published four years after the book that succeeds it in the main series, brings some characters into the Abbey series (Betty Maclean from Abbey Girls At Home onwards and Rena Mackay and Lisabel Durrant, who also appear in Rosamund's Victory). To appreciate the background to these characters, A Go-Ahead Schoolgirl (for Rena and Lisabel) and Tickles (for Betty) should be read before Jen of the Abbey School.
The only other series with a direct connections are the Torment Series and the Swiss Set. Tormentil Grant, Tazy Kingston and Karen Wilson appear in Abbey Girls Go Back to School. This was published two years after the first Torment book, A School Torment (1920). Two more Torment titles would appear in 1925 and 1926. In 1921 Two Form Captains had been published, introducing Tazy and Karen, as well as Girls of the Abbey School, so it is evident that EJO was working on these titles at about the same time, if not all at once! Karen is mentioned again in AG At Home, and Tazy reappears in Abbey Champion.