Any book published in the US prior to 1923 is in the public domain in the US, so that's just the two Agatha Christie's that you've found.
In most of the rest of the world, books enter the public domain at the start of the year following either the 50th or the 70th anniversary of the author's death. Agatha Christie died in 1976, so the rest of her books won't enter the public domain in "life + 50" countries until 2027, and in "life + 70" countries until 2047. I'll leave you to check the other authors.
Over 50 of Agatha Christie's books are, however, available as very reasonably-priced eBooks at any decent eBook store - eg FictionWise.