This is the 3414th most frequent Turkish word.
koyduğunu
"That which (someone) placed/put."
Here, 'koyduğunu' refers to the act of putting something (a note) into the book.
Kitaba not koyduğunu fark ettim.
I noticed that you put a note in the book.
In this sentence, 'koyduğunu' relates to the placement of the book in a specific location, in this case, the library.
Kütüphaneye koyduğunu söylediğiniz kitabı arıyorum.
I am looking for the book you said you put in the library.
'Koyduğunu' in this context describes the assumed action of placing the bag on a table.