I looked into this when I bought the ebooks in a fictionwise sale in 2009. It's complicated a bit by different titles and even slightly different US and UK publications, but I put together a list of the books. The numbered ones are the current ebooks. Unnumbered ones are original (usually US) publications that are included in later books. First publication date is given. Alternative titles are given in parenthesis. Main characters follow the titles.
"The Mousetrap" short story isn't in any of the ebooks, as far as I know. It still can't be sold in the UK, as UK publication of the story isn't allowed until the end of the run of the stage play of the same name. It's been running continuously since 1952 at St. Martin's Theatre, and the run shows no sign of coming to an end...
Spoiler:
1 1920 The Mysterious Affair at Styles Hercule Poirot
2 1922 The Secret Adversary Tommy and Tuppence
3 1923 The Murder on the Links Hercule Poirot
4 1924 The Man in the Brown Suit Anne Beddingfeld
5 1924 Poirot Investigates (short stories: eleven in the UK, fourteen in the US)
6 1925 The Secret of Chimneys Anthony Cade
7 1926 The Murder of Roger Ackroyd Hercule Poirot
8 1927 The Big Four Hercule Poirot
9 1928 The Mystery of the Blue Train Hercule Poirot
10 1929 The Seven Dials Mystery Eileen "Bundle" Brent
11 1929 Partners in Crime (fifteen short stories; featuring Tommy and Tuppence)
12 1930 The Murder at the Vicarage Miss Marple
13 1930 The Mysterious Mr. Quin (twelve short stories; Mr. Harley Quin)
14 1931 The Sittaford Mystery (Murder at Hazelmoor)
15 1932 Peril at End House Hercule Poirot
16 1932 The Thirteen Problems (The Tueday Club Murders) Thirteen short stories; Miss Marple.
17 1933 The Hound of Death (twelve short stories – UK only)
18 1933 Lord Edgware Dies (Thirteen at Dinner)
19 1934 Why Didn't They Ask Evans? (The Boomerang Clue)
20 1934 Murder on the Orient Express
21 1934 Parker Pyne Investigates (Mr. Parker Pyne, Detective) Twelve short stories; Parker Pyne and Ariadne Oliver.
22 1934 The Listerdale Mystery (twelve short stories – UK only)
23 1935 Three Act Tragedy (Murder in Three Acts)
24 1935 Death in the Clouds (Death in the Air)
25 1936 The A.B.C. Murders
26 1936 Murder in Mesopotamia Hercule Poirot
27 1936 Cards on the Table Hercule Poirot
28 1937 Dumb Witness (Poirot Loses a Client) Hercule Poirot
29 1937 Death on the Nile Hercule Poirot
30 1937 Murder in the Mews (Dean Man's Mirror) (four stories; Hercule Poirot, In the US missing the story The Incredible Theft.)
31 1938 Appointment with Death Hercule Poirot
32 1938 Hercule Poirot's Christmas (Murder for Christmas/A Holiday for Murder)
33 1939 Murder is Easy (Easy to Kill)
34 1939 And Then There Were None (Ten Little Niggers/Ten Little Indians)
1939 The Regatta Mystery (US Only)
35 1940 Sad Cypress Hercule Poirot
36 1940 One, Two, Buckle My Shoe (The Patriotic Murders/An Overdose of Death)
37 1941 Evil Under the Sun Hercule Poirot
38 1941 N or M? Tommy and Tuppence
39 1942 The Body in the Library Miss Marple
40 1942 The Moving Finger
41 1943 Five Little Pigs (Murder in Retrospect)
42 1944 Towards Zero Superintendent Battle
43 1944 Death Comes as the End Hori
44 1945 Sparkling Cyanide (Remembered Death)
45 1946 The Hollow (Murder After Hours)
46 1947 The Labours of Hercules (twelve short stories; Hercule Poirot)
47 1948 Taken at the Flood (There is a Tide)
1948 Witness for the Prosecution and Other Stories (US Only, already in UK collections)
48 1949 Crooked House Charles Hayward
1950 The Mousetrap and Other Stories (US Only)
49 1950 A Murder is Announced Miss Marple
50 1951 They Came to Baghdad Victoria Jones
1951 The Under Dog and Other Stories (US Only, included in Poirot's Early Cases)
51 1952 Mrs McGinty's Dead
52 1952 They Do It with Mirrors (Murder with Mirrors)
53 1953 A Pocket Full of Rye Miss Marple
54 1953 After the Funeral (Funerals are Fatal)
55 1954 Destination Unknown (So Many Steps to Death)
56 1955 Hickory Dickory Dock
57 1956 Dead Man's Folly Hercule Poirot
58 1957 4.50 from Paddington (What Mrs. McGillicuddy Saw)
59 1958 Ordeal by Innocence
60 1959 Cat Among the Pigeons Hercule Poirot
61 1960 The Adventure of the Christmas Pudding (six short stories – UK only)
1961 Double Sin and Other Stories (US Only, included in other UK collections)
62 1961 The Pale Horse Inspector Lejeune
63 1962 The Mirror Crack'd from Side to Side (The Mirror Crack'd)
64 1963 The Clocks Hercule Poirot
65 1964 A Caribbean Mystery Miss Marple
66 1965 At Bertram's Hotel Miss Marple
67 1966 Third Girl Hercule Poirot
68 1967 Endless Night
69 1968 By the Pricking of My Thumbs Tommy and Tuppence
70 1969 Hallowe'en Party Hercule Poirot
71 1970 Passenger to Frankfurt
1971 The Golden Ball and Other Stories (US Only)
72 1971 Nemesis Miss Marple
73 1972 Elephants Can Remember Hercule Poirot
74 1973 Postern of Fate
75 1974 Poirot's Early Cases (Hercule Poirot's Early Cases)
76 1975 Curtain
77 1976 Sleeping Murder
78 1979 Miss Marple's Final Cases and Two Other Stories (eight short stories – UK only)
79 1991 Problem at Pollensa Bay and Other Stories (eight short stories – UK only)
80 1997 While the Light Lasts and Other Stories (nine short stories – UK only)
42 1944 Towards Zero Superintendent Battle
43 1944 Death Comes as the End Hori
44 1945 Sparkling Cyanide (Remembered Death)
45 1946 The Hollow (Murder After Hours)
46 1947 The Labours of Hercules (twelve short stories; featuring Hercule Poirot)
47 1948 Taken at the Flood (There is a Tide)
1948 Witness for the Prosecution and Other Stories (US Only, already in UK collections)
48 1949 Crooked House Charles Hayward
1950 The Mousetrap and Other Stories (US Only)
49 1950 A Murder is Announced Miss Marple
50 1951 They Came to Baghdad Victoria Jones
1951 The Under Dog and Other Stories (US Only, included in Poirot's Early Cases)
51 1952 Mrs McGinty's Dead
52 1952 They Do It with Mirrors (Murder with Mirrors)
53 1953 A Pocket Full of Rye Miss Marple
54 1953 After the Funeral (Funerals are Fatal)
55 1954 Destination Unknown (So Many Steps to Death)
56 1955 Hickory Dickory Dock
57 1956 Dead Man's Folly Hercule Poirot
58 1957 4.50 from Paddington (What Mrs. McGillicuddy Saw)
59 1958 Ordeal by Innocence
60 1959 Cat Among the Pigeons Hercule Poirot
61 1960 The Adventure of the Christmas Pudding (six short stories – UK only)
1961 Double Sin and Other Stories (US Only, included in other UK collections)
62 1961 The Pale Horse Inspector Lejeune
63 1962 The Mirror Crack'd from Side to Side (The Mirror Crack'd)
64 1963 The Clocks Hercule Poirot
65 1964 A Caribbean Mystery Miss Marple
66 1965 At Bertram's Hotel Miss Marple
67 1966 Third Girl Hercule Poirot
68 1967 Endless Night
69 1968 By the Pricking of My Thumbs Tommy and Tuppence
70 1969 Hallowe'en Party Hercule Poirot
71 1970 Passenger to Frankfurt
1971 The Golden Ball and Other Stories (US Only)
72 1971 Nemesis Miss Marple
73 1972 Elephants Can Remember Hercule Poirot
74 1973 Postern of Fate
75 1974 Poirot's Early Cases (Hercule Poirot's Early Cases)
76 1975 Curtain
77 1976 Sleeping Murder
78 1979 Miss Marple's Final Cases and Two Other Stories (eight short stories – UK only)
79 1991 Problem at Pollensa Bay and Other Stories (eight short stories – UK only)
80 1997 While the Light Lasts and Other Stories (nine short stories – UK only)