What was (or is?) the difference between a between-maid and a parlor-maid? Both are mentioned in some Agatha Christie books.

The parlour maid was the more senior of the two; they kept reception rooms tidy, served afternoon tea, sometimes dinner too, and answered room service bells etc. The between maid was similar to a scullery maid (a junior position and not well paid). She would have waited on the more senior servants. Section 2 of this link explains more:

https://en.wikipedia.org/wiki/Maid

Parlour maid - the parlour maids cleaned and tidied reception rooms and living areas in the mornings and often served refreshments at afternoon tea and sometimes also served dinner. They tidied studies and libraries and (with footmen) answered bells calling for service.

Between maid - roughly equivalent to scullery maids (dishwashers, floor scrubbers, oven minders,so etc.) and often paid less. The between maids in a large household waited on the senior servants (Butler, Housekeeper, Cook) and were therefore answerable to all three department heads, often leading to friction in their employment.

(Pretty much what I do every day around here in my own house!)

(also from https://www.merriam-webster.com/dictionary/betweenmaid)