https://courses.dcs.wisc.edu/wp/readinggerman/introduction/: "Einfluß" and "Zusammenfluß" have officially been spelled "Einfluss" and "Zusammenfluss" since the reform of 1996, i.e., since before much of this book's audience was born. "beeinflüssen" should be "beeinflussen".
https://courses.dcs.wisc.edu/wp/readinggerman/noun-gender-no...: Not an error of German per se, but I find "memorizing the gender of every noun is not particularly important" and "It is recommended that, as you learn the nouns you choose to memorize, you learn each noun with its definite article" contradictory and confusing.
On the same page: "All nouns that end in –ei, –heit, –ie, –in, –keit, –schaft, –tät, –ung are feminine." Counterexamples I came up with in a minute or so: der Brei, das Allerlei, das Benzin. The reason is that in these cases -ei and -in are not suffixes in the same sense as they are in the common case illustrated by the authors' examples, but a beginner would not be able to tell.
This feels like it could be a better resource with more attention to small details.