So, in order to test the hypothesis "Silmarillion is harder to read because it has lots of stop words", you need to calculate the relative frequencies of lots of other texts and see if there's something special about Silmarillion's top 10 versus all other's.

Surely, you have already done that using LOTR and The_Hobbit, but a much bigger sample is needed. At the very least, you may want to use 10-15 other works of fantasy from different authors, and that will be just like a back-of-the-envelop test to see if it is worth to pursue this experiment with a statistically significant sample.

[edit] 1. Provided it is sufficiently large.