Project Gutenberg is one of the more popular sources of ebooks.

At present it has a listing of more than 30,000 ebooks.

They are available in different formats like

HTML

EPUB (with images)

EPUB (without images)

Kindle (with images)

Kindle (without images)

Plain text UTF-8

The file sizes for some of books are smallest for EPUB (without images)  and  largest for  Kindle (with images)