Take a look at Calibre as an excellent, and free, book manager. While it can not open or save metadata to DRM infested books, it will still manage them and put them on your reader for you. It's a fantastic resource for large libraries no matter what reader you get. I'd also look at the comparison matrix located at
https://www.mobileread.com/eink/, as it will show you less commonly known readers too. All the books that I've handled have been recognized as an external hard drive when attached to a computer via USB cable.
In general, I'm aware of three management schemes. The first group of readers only allow you to list books by author or title. These manufacturers appear to believe that you will purchase all of your books from them, keep them on their servers, and only download them a few at a time as you read them. The Nook is in the camp now, as was the Kindle until the most recent model.
The second group of manufacturers use folders to organize. You can have as many folders and subfolders as you like on these devices, but you have to have an actual copy of the book in a folder if you want to find the book there. This can be a surprisingly good way to organize a library if you're using Calibre. You tell Calibre what information you want to use to create the folder structure. When you save to disk, Calibre using that information to make your folders. Calibre is smart enough to recognize that you don't want several copies of the same folder, so it won't recreate the folder if you save more books there later on. This has the advantage of letting you put everything exactly where you want it. I wanted author as the top folder and series folders next if there was a series. I also wanted the series number at the beginning of each title. Finding unread books was important to me, so my top folders were Read and Unread. Calibre did all of this for me automatically. The negative is that the folders can become cluttered, especially if you have the same book in different folders.
Third are a few readers that use collections. Sony has had these for several years and the Kindle 3 has them now as well. I can't speak for the Kindle, but the Sony uses metadata to assign books to Collections. I use Calibre to add tags and series information to each book and Calibre creates collections based on the tag and series information. The plus is that I can have one book in several different categories. For instance, I might tag a Sherlock Holmes book with Detective, historical fiction, and 19th century Britain and add it to the Sherlock Holmes series. The plus is that when Calibre sends that book to the device, it will create these collections if they don't already exist and then add the book to them. I can open the book from any of those collections, but they're all tied to the same file on the reader, so information like place in the book and bookmarks is the same no matter where on my reader I look for the book. When I delete the book from the reader, it disappears from all of the collections at the same time. The negative with this system is that I can't construct a search that uses more than one criteria at a time. I can't, for instance, decide that I want to find a detective novel that's historical fiction. I have to look in one of those two categories and try to identify ones that meet the other criteria at the same time.
You may have noticed that I use Calibre for both types of reader organizations. That fact alone makes 3G and WiFi useless to me. It's way more important to me that Calibre manage putting books on my reader and it's not hard to attach it to the computer with a cable. In fact, I'd argue that if I'm in the same building as my computer, it's easier to plug my reader into the computer than it would be to navigate to them in an online account, and Calibre manages them for me way better than either Sony or Amazon.
Also recognize that there are lots of passionate people here who know that their device is the best device. I recommend that you use the matrix to see what's out there and decide what's most important to you. Once you know that, you can see which readers come closest and then start looking at specifics.