Didnt work for me, upon clicking the icon nothing happens, i cleared cache and disable addblock, on the console when i click it, i get "Error: NetworkError when attempting to fetch resource."

Edit: Thanks for the reply, yes I am using the main domain, tried just with ebooks.
Update: I tried it on Chrome, it worked there without issues.

Edit: To the developer; thank you for your response! It was an issue with my eReader that I have not encountered before. I don't know why that did not occur to me before- it seems like some updates I did stopped my reader from working for epubs. Strange, but anyway! The downloaded file is perfect, it includes links and images (which is really important to understand this specific book). I'm glad a tool like this is available otherwise I would not have been able to read O'Reilly books comfortably. Sorry for the confusion caused!

Original review below:
I tried using this extension to download a book but it seems like the downloaded file was corrupted because I cannot open it. I tried twice with the same result. Not sure if this is a bug or an issue caused by O'Reilly updating something in their system. Either way... I couldn't make use of it which was disappointing.

Surprisingly it works, which is great! It was basically exactly what I needed. A few issues though: it wasn't obvious to me that the file will be downloaded automatically, I downloaded it a few times until I realized that it had downloaded in the download menu (perhaps this is more of a Firefox problem, but if you could do anything about that, that would be great); also the screen where it says "the assets you download will be listed here for reference", doesn't work, it doesn't show anything; finally, it would be nice if it would be possible to download directly as a pdf, instead of an epub. All in all, great add on though!

Doesn't download the complete book.
When the epub is downloaded, it only contains all book sections; for instance Chapter 1 [sub-topics,..], 2, 3 and so on, but when clicking on each sub-topic, it doesn't display any information. This happens with all books.

Posting URL
https://learning.oreilly.com/library/view/ccnp-and-ccie/9780135262047/part7.xhtml

I see, I guess I am wrong, my bad.
I am using E-book viewer in Ubuntu.
I will try it with another viewer.

it worked as expected for English contents of Oreilly but epub format was corrupted for Korean contents of Oreilly.

Since I cannot reply to your response, I edited my review instead. I tested these two books https://learning.oreilly.com/library/view/kubeonetiseureul-hwalyonghan-keulraudeu/9791162248249/ and https://learning.oreilly.com/library/view/keulraudeu-neitibeu-keulraudeu/9791162243183/ . This extension successfully downloaded epub files for both but the epub contents formats were corrupted with very big and overlapped fonts for both.

Does what is says it does and it does a pretty good job at it. Saved me from loosing my mind from using the original android app to listen to an audio book. Thanks!