I tried doing this mission myself without spoilers and found the answer to this question. At the beginning of the mission, Angus says that someone in Ab'Dendriel is definitely in possession of a copy of the book 'Songs of the Forests'.
Following that lead, I asked some NPCs if they knew anything about the book. The right person turned out to be Elvith. He told me where I should look for this book.
Elvith told me where to look for the book, but advised me against going to hellgate. So there should be another way to get the book. If you have pulled a book from hellgate before then you can buy a copy.
Elvith offered to resell me a copy of this book. Well, and I bought several copies of the 'Songs of the forsest'
Back to your question, to get more than one copy of Elven Poetry Book we need to buy them from Elvith during the quest after pulling the book from the chest on hellgate. The keyword when buying the book is 'Elven Poetry'. The book costs 500gp and we can only buy it during the quest. Unfortunately, once you give the book to the mission giver, Elvith will no longer have a copy of the book.
Transcripts:
20:11 Mogh [344]: Songs of the Forest
20:11 Elvith: The last issue I had was bought by Randor Swiftfinger. He was banished through the hellgate and probably took the book with him ...
20:11 Elvith: I would not recommend seeking him or the book there, but of course it is possible.
20:11 Mogh [344]: Elven Poetry
20:11 Elvith: By luck I acquired another copy of the book you are looking for. Do you want to buy a copy of 'songs of the forest' for 500 gold?
20:11 Mogh [344]: yes
20:11 Elvith: Thank you. Here it is.
Sources:
https://tibiopedia.pl/quests/The_Explorer_Society_Quest
https://tibia.fandom.com/wiki/Elven_Poetry_Book