Dew's Agatha
Christie Mystery Site
Agatha Christie Character Tie-Ins
Hercule Poirot==>Ariadne Oliver==>Mrs. Dane Calthrop==>Miss Marple
Mrs. Oliver==>Parker Pyne
Mrs. Oliver==>Hercule Poirot
(Mrs. Oliver was employed at one time by Mr. Parker Pyne and she knows Hercule Poirot.)
Mr. Sattherthwaite==>Hercule Poirot
Mr. Sattherthwaite==>Harley Quin
(Mr. Sattherthwaite, who appears with Hercule Poirot in Murder in Three Acts, is also an acquaintance of Harley Quin.)
Cards On The Table = Poirot + Race + Oliver + Battle
Battle's Children:
Sylvia (Towards Zero)
Colin Lamb (The Clocks)
Battle is married and has five children. One of them a daughter Sylvia is in Towards Zero and it is also believed that Colin Lamb (The Clocks) is a son of Battle's.
durmaz_murat also submitted this info:
Superintendent Battle: Appears in his own detective books and also Poirot's
Dane Calthrop the Reverend and Maude: Miss Marple and Ariadne Oliver
Hugh (or John) Despard and Rhoda Dawes Despard: Poirot and Ariadne Oliver
Boris Ivanovitch: Tommy and Tuppence, and Poirot
Julia Lancaster: Tommy and Tuppence. A person strangely similar is also referred to in a Miss Marple novel (Sleeping Murder) and an Ariadne Oliver novel (The Pale Horse).
Lemon Felicity: Poirot and Parker Pyne
Colonel Melrose: Two Battle novels and one Harley Quin short story
Oliver Ariadne: Poirot and Parker Pyne
Pikeway Colonel: Poirot, Tommy and Tuppence plus (I think) Passenger to Frankfurt
Johnny Race: Poirot (two novels) and Sparkling Cyanide
Mr. Robinson: Poirot, Marple and Tommy and Tuppence
Mr.Satterthwaite: Harley Quin and Poirot (Three Acts)
(Information from members of agatha_agape and various books)