<<outline-container-sec-1>> tl;dr Bottom line: If you want to read a good translation of Newton's Principia, you almost certainly want Cohen and Whitman's: Barnes & Noble Amazon

<<text-1>> In 1687, Isaac Newton published Philosophiæ Naturalis Principia Mathematica, a foundational work of science originally written in Latin. Finding an English translation turned out to much harder than I expected.

A simple Google search for "newton's principia" turns up unsurprising links on the first page: the Wikipedia entry, a scanned ebook at archive.org, the SparkNotes summary for those interested in cramming for the test, an entry in the Stanford Encyclopedia of Philosophy (don't start clicking links here if you have something else to get done), a link to Amazon (not linked here for reasons that will become clear), a one-paragraph item on Pearson's abbreviated Infoplease, a color scanned copy of Newton's own copy of the first edition at the University of Cambridge's digital library (which the detailed description understates as "with manuscript notes by the author!), a Project Gutenberg ebook of the first edition, and a page on Principia in an online exhibition from the University of Sydney of important works in the evolution of modern thought.

As it turns out, the most useful thing would probably have been to skimmed through the Wikipedia article to find out about translations, but it didn't occur to me at that point that the answer would be so complicated. A quick search for "newton principia pdf" turned up links to various scanned versions of the Latin editions and some old translations to English. Having failed to quickly find something readable, I searched both Barnes & Noble and Amazon. That's where the search gets to be extraordinarily confusing, because of various editions in Latin and translations into English.

The first edition was completed in 1686 and approved by Samuel Pepys (then President of the Royal Society) for publication, which actually occurred in 1687 (the date in the front matter is the day Pepys gave his imprimatur for it). According to Wikipedia, Edmund Halley (then clerk of the Society) paid for the publication because the Royal Society had already used up its publication funds for Francis Willughby's De Historia Piscium (The History of Fish). It also turns out that later Halley was not paid for his clerkship as promised by the Royal Society, but was paid with copies of/The History of Fish/. Newton also published a second edition in 1713, and a third in 1726, shortly before Newton's death. The third edition was thoroughly revised from the first, although most of the changes came between the first and the second. The Stanford Encyclopedia of Philosophy has a discussion of some of the differences, as does I. Bernard Cohen's A Guide to Newton's Principia that accompanies his translation.

All of those editions were in Latin. The Principia has been translated into English only a few times for publication: by Andrew Motte in 1729, by Robert Thorp (Book 1 only) in 1777 with a second edition in 1802, along with a version of Motte's partially revised by Florian Cajori published in 1934, and modern translation by I. Bernard Cohen and Anne Whitman in 1999. Motte's translation was revised by N. W. Chittenden for the first American edition in 1846 (scanned copy). There is also an online translation by Dr. Ian Bruce at 17th Century Maths, and there have also been various translations of portions of Principia.

In addition to the scanned Latin texts already mentioned, the definitive Latin edition now was edited by Alexandre Koyré and I. Bernard Cohen with Anne Whitman, and published by Harvard University in two volumes in 1972 as Isaac Newton's Philosophiae Naturalis Principia Mathematica: Facsimile of third edition (1726) with variant readings. The accompanying Introduction to Isaac Newton's "Principia" is available as a reprint e-book.

Cohen critiques both Motte and the Motte-Cajori translations the Guide. Cohen also wrote an Introduction to Newton's Principia, originally published by Harvard University Press. It is hard to find, but there is a 2013 reprint, as well as used book stores. The Harvard Gazette published a short article about Cohen and Whitman's translation when it was published by the University of California Press in 1999.

So all of this explains why there's not a good English version available online–there's really only one choice, and it's Cohen and Whitman's. But that's not the end of the problem: it's hard to figure all this out on either Amazon or Barnes & Noble. Both sites conflate various of these versions in reprints together, including the comments, so it's not at all obvious what you're getting. The descriptions and the comments don't always refer to exactly what is displayed. Some of the commenters have been kind enough to specify exactly what they are talking about, so there are some clues. But beware the one-click purchase here!

Bottom line: If you want to read a good translation of the Principia, you almost certainly want Cohen and Whitman's: Barnes & Noble Amazon

Bonus link: Brent Nordist has his own notes In Search of Principia about finding a free online translation.