This is the 388th most frequent Turkish word.
geldiği
"geldiği" means "that he/she/it came" or "where he/she/it came from," depending on context.
In this sentence, 'geldiği' is used as part of a time clause indicating 'the time when he came/comes'.
O, buraya geldiği zaman hep hediyeler getirir.
When he comes here, he always brings gifts.
In this case, 'geldiği' describes the origin or source related to 'the person from whom the book came'.
Bu kitabı bana geldiği kişi hediye etti.
The person from whom I received this book gifted it to me.
Here, 'geldiği' refers to the act of 'his/her coming' in a context indicating past or completed action.