Does anyone know where I can find the posthumously published (I think) chapter 8 of Gauss's Disquisitiones Arithmaticae?
Go to archive.org and look up Gauss' Werke, Band 1. This is in German and it includes the unfinished notes that would have become part of Section 8. But I would strongly recommend reading Mathews book on number theory first because it attempts to go over the content of Gauss' DA in a more up-to-date and accessible fashion.
Maser's 1889 German translation has on the title page: "the last third of the text contains Gauss's published papers on The Theory of Numbers, followed by his posthumous writings on that subject". It's about 230 pp., tacked onto books I-VII of Disquisitiones. It's available as a reprint (mine is Chelsea press). I don't know if this is considered to be the missing book or not ... is there consensus about what the proper content of book VIII would have been?