Note: I HIGHLY suggest using an adblocker on this site and the sites it might link to.
Go to Amazon and search up the textbook you're looking for, then open up its listing. Make sure it's the correct edition.
Scroll down until you find the ISBN (under Product Details) and copy it.
Go to Library Genesis and paste the ISBN for your textbook into the search bar. Keep in mind the Extension column in the search results. This will tell you what file type you are downloading. Typically it will be one of two:
EPUB: very nice file type with lots of metadata. You'll be able to see chapters and such very easily. This file can be opened with an epub reading software, or with a Kindle. Not as easy to work with on PC.
PDF: usually has less metadata but much broader compatibility. You can open these suckers very easily, without even downloading any additional software. I usually gravitate towards PDFs because I can open them in my browser.
Whichever result you would like to download, underneath the Mirrors section, click on #1.
From here, click the big Get link at the very top of the page
This will initiate the download. Enjoy!
If for whatever reason the first mirror does not take you to this page, instead of clicking on #1, open the search result itself and go through a couple of the listings in the Mirrors section until you find one that downloads the book for you.