This is the 12740th most frequent Turkish word.
istediğinde
"When (he/she/it/you) wants."
In this sentence, 'istediğinde' is used to indicate a specific point in time defined by Ahmet's willingness or decision to join.
Ahmet istediğinde bize katılabilir.
Ahmet can join us whenever he wants.
'İstediğinde' here implies an open invitation to act (to knock on the door) determined by the person's wish.
İstediğinde kapıyı çal ve bekle.
When you want, knock on the door and wait.
The usage of 'istediğinde' in this context shows an action (reading books) occurring based on the subject's desire.