I elaborated this idea as follows:
Input format of #readstatus: person (date). If I dropped reading of certain book, I just enter my name, like NQK!.
I created another composite column:
{#readstatus:contains(NQK \(, Yes, {#readstatus:contains(NQK!, Dropped, Pending)})}
It works.

. The result is a column with three values: Yes (books I have read), Pending (books I haven't read) and Dropped (books that I didnot finish intentionally).