It wasn't all that long ago here in cyberspace that tracking down anything of substance written by Zwingli in any language was futile, let alone English. Now Google books and a few other web sites have helpfully put up some of his works translated into English, most particularly,
The Latin Works and the Correspondence of Huldreich Zwingli, Volume 1
The Latin Works and the Correspondence of Huldreich Zwingli, Volume 2
There is also a third volume to this series, but as of the writing of this blog entry, it isn't available on Google Books. Even when I looked around for a used copy of volume 3, there were only a few I was able to track down. I spent a few minutes trying to determine the contents of Volume 3, and it appears to include at least an English translation of Zwingli's Commentary on True and false Religion. While this treatise isn't online, it still is available in print.
Logos software has recently made available these three volumes with a few others in their recent software package, The Works of Zwingli (7 Vols.). The other four volumes in this set are as follows:
Huldreich Zwingli: The Reformer of German Switzerland (available on Google Books).Logos wants $70 for this package, and if you're like me, you're wondering why you would want to spend $70 on 7 books when 5 of them are already readily available for free. I have a fairly large collection of Logos books, and I appreciate the digital format. But, I also appreciate not spending $70 on books I really don't need to spend $70 on. Jim West's book is affordable. The only part of this Logos package that's worth it is Volume 3 of the old Latin Works /Correspondence set. But here's the thing: I've jumped the gun in the past and bought a pricey old book because it wasn't available online, only to find that book free online maybe a few months later. I won't admit how often I've done this, but let's just say it's been more than once. For me, this Logos Zwingli set just isn't worth it.
Zwingli: The Christian Education of Youth (available on Google Books)
Life of Ulrich Zwingli, the Swiss Patriot and Reformer (available on Google Books)
'Christ Our Captain': An Introduction to Huldrych Zwingli by Jim West (not available on Google Books, but available for a very reasonable price here. The author's blog can be found here, as is his description of working with Logos on this package).