This is the 10470th most frequent Polish word.
czytelniczkom
"Czytelniczkom" means "to/for (female) readers" in English.
In this sentence, 'czytelniczkom' (dative plural form of 'czytelniczka' – female reader) is used as the indirect object of the verb 'podziękował' (thanked), indicating who the thanks are directed towards.
Autor książki podziękował swoim czytelniczkom za wsparcie.
The author of the book thanked his female readers for their support.
Here, 'czytelniczkom' serves as the recipient of the direct object's (free copies) action, marked by this prepositional phrase to show to whom it is sent.
Wydawnictwo przesłało darmowe egzemplarze swoim wiernym czytelniczkom.
The publishing house sent free copies to its loyal female readers.
In this case, 'czytelniczkom' is used to specify the group of individuals for whom the event was particularly intended.