The books in the Zeltora heptalogy hold many obscure secrets. One of those secrets is that there is music placed inside every 

book, called the "Battle Themes". These are codes hidden throughout the margins of each book, which correlate with the 

storyline and contain lore from within the story in one way or another. The Battle Themes links are hidden but they

 are simple to unlock once you know the secret. All you have to do is find the one of the eleven digit long codes, 

(here is an example -BPxwfZwnLQ), now take this code (just pretend you found it in the book for a moment), 

and paste it directly after an empty YouTube link. It should look exactly like the example shown below.

https://www.youtube.com/watch?v=-BPxwfZwnLQ

A blank YouTube link (white) followed by a Battle Theme code (bright green) without a space in between.

Once you have copied the link shown above into a new tab, (like you just did if you were following along with this example)

 copy the Battle Theme directly after it (like we just did in bright neon green) and press enter.If you followed these 

very complicated and incredibly difficult steps correctly then the link you just inserted should lead you to a 

video titled, "Captain of the Guard - An Undertale Fan Short". If the link failed to work or took you

 somewhere else on the internet, then you definitely did something wrong or missed one of these steps. 

If you succeeded, you will now know how to use secret codes to find Battle Themes all on your own.

The hardest part to finding the Battle Themes is just locating the three codes that I have hidden in each book. 

Without an actual code, a Battle Theme is just an empty base YouTube link that doesn't really lead to 

anything useful. Also, there are always three additional codes hidden in every other book.

Please enjoy.

P.S. If you're lazy, and don't want to actually look for the hidden Battle Themes, all the codes are listed right here for ease of access, but just remember these may not be the finalized versions of the Battle Themes... Yet. They may be misleading if you listen to them ahead of time. Why would you cheat in a battle theme code when it's more fun to actually re-read the story again to find it yourself? That's specifically how I designed the Battle Theme puzzle to work. Also note that the Battle Themes are not officially canon to the Zeltora heptalogy, but they may contain hidden lore and untold information. Additionally, the videos are not specifically created for any of the Battle Themes. They only exist for sparking the imagination of the reader and fully belong to their respective owners. Because of this, the Battle Themes are only semi-cannon, but they are still fun to listen to and interpret. Also note these may not be the finalized versions of the Battle Themes and they could change over time. Please do not think I am in any way attempting to remotely pirate other creators content. I did not create the videos from any of the Battle Themes nor do I have any claim on their creation. So, please go support the actual creators on YouTube if you liked any of these Battle Themes.

Note: After some thought I have decided not to list any of the "unpublished" Battle Themes here yet, otherwise people would just look ahead.