I've found this great website today! You can download big document collections as ZIM files from there and view these documents by a desktop app or a web server on localhost. Maybe not as good as compiling an own collection from the original sources like I did with the Gutenberg Project, but it definitely saves you time. E.g. I can download part of #Wikipedia on #mathematics or #books from the Gutenberg Project on #socialsciences #medicine or #philosophy and #religion