HMMM. You had to pick a rare book, didn't you?
As a "portal" for finding downloadable ebooks, here is a REALLY great source, they have a very large database with links to many sources, including gutenberg, archive.org,, JStor.org, and many many more.
http://onlinebooks.library.upenn.edu
I did look on there for your Chesterton book, and there is only one link in addition to the PGutenberg, at the Christian Classics Ethereal Library. That site needs a free registration to download, or freely accessible to read online. A disclaimer for "Trees of Pride" says "this book not proofed yet" so it might just be another Gutenberg version.
https://www.ccel.org/
[s]
Jstor.org[/s] (OK, strikethrough is not working)I MEANT HATHI-TRUST
http://babel.hathitrust.org
has lots of pdfs [but I didn't see any for Trees of Pride], but for many of their pdfs you have to have a "partner log-on" from a university or other facility to download the entire book as a single file. If you don't have a partner log-on, you can only download one page at a time. Ok for magazine articles, but not for a 300-page book!
OOOHHH! I know! Wikisource!!!! I almost forgot about them.
http://en.wikisource.org/wiki/The_Trees_of_Pride
It is not annotated as being "in progress", so proof-reading has most likely been completed. Give it a peek.