Where does “पुस्तिका” come from?

पुस्तिका (Sanskrit) comes from Sanskrit पुस्तक, from Proto-Indo-Aryan pōstaka.

Ancestry of “पुस्तिका”, step by step

Step	Language	Word	Meaning
1	Sanskrit	पुस्तक	a protuberant ornament, boss; book, manuscript,...
2	Proto-Indo-Aryan	pōstaka	—

पुस्तिका — Etymology, Origin & Word History | Etymology Explorer