YA Fantasy series where the friend of the main character leaves a hint "square root" about the villain... Spoiler.

[Bori Lee]

I have no recollection of the villain's motivations but a friend of the mc is for some reason unable to tell him directly where the villain is hiding, so she writes 4, 9, 16, 25, 36 ie; square root which was supposed to lead him to a giant tree root that was literally in the shape of a square. some other things i remember are that the father of the mc is a lotr fan and the mc uses that to quiz him and check if he's a fake lol, also "you should never call a dragon a worm"? and this miiight be a different book but i remember hating this scene near the end of the series where the main crew just barely escapes through a secret tunnel through a fireplace but they get caught because the villain's right hand man has a special ability that just entails "seeing secret passageways".

i think i read this sometime around 2015-2016? hope this is enough pf a distinguisher for someone to recognize it :)

ah also i dont trust my memory on this but i think the book opens with the main character talking to/whistling at..? a nest of birds
/eagles, and this comes back into play later .. for some reason. sorry, i feel like a couple of these bits might be from different books because i read so many similar fantasy novels as a kid.