Howdy Hadrien.
I tried to d/l a couple of originally-project-Gutenberg books from your site and noticed that they'd benefit a lot from postprocessing by e.g., the
GutenMark script -- otherwise, you have things like "Mr." and "Mrs." treated as last words in a sentence, en-dashes where em-dashes should have been, emphasized words via all-capitals, irritating right-double-quotes instead of both left and right ones, etc.
Otherwise, your site is great. I especially like the customization feature, allowing one to select their own typeface/size.