The best place to look for public domain ebooks to produce is Project Gutenberg. If downloading from Project Gutenberg, be careful of the following:
There may be different versions of the same publication on Gutenberg, and the best one might not be the one with the most downloads. In particular, there could be a better translation that has fewer downloads because it was produced later, or there could be a version with better HTML markup. A great example of this phenomenon is the Gutenberg version of 20,000 Leagues Under the Seas. The most-downloaded version is an old translation widely criticized as being slapdash and inaccurate. The less popular version is a fresh, modern translation dedicated to the public domain.
+There may be different versions of the same publication on Gutenberg, and the best one might not be the one with the most downloads. In particular, there could be a better translation that has fewer downloads because it was produced later, or there could be a version with better HTML markup. For example, the version of Journey to the Center of the Earth with the most downloads is a less accurate translation than its less-frequently downloaded counterpart. However, you must verify that whichever version you ultimately select is not copyrighted. For example, this modern translation of Twenty Thousand Leagues Under the Seas compares favorably to the most-downloaded older translation, but because the modern translation is copyrighted (see the disclaimer at the top of the HTML file) it is ineligible to be the basis of a Standard Ebooks production.
Gutenberg usually offers both an HTML version and an epub version of the same ebook. Note that one is not always exactly the same as the other! A casual reader might assume that the HTML version is generated from the epub version, or the other way around; but for some reason the HTML and epub versions often differ in important ways, with the HTML version typically using fewer useless CSS classes, and including <em> elements that the epub version is often missing.