I read on an Xteink X4 Pro, a $99 pocket e-reader with a 4.3" E Ink touchscreen, a warm and cool front light, 2.4 GHz Wi-Fi and a 16 GB microSD card. Most of what I read on it is a book that keeps growing: new chapters get appended every few days, and I want to pick up where I left off. That takes a server on the other end, and there are four common kinds: OPDS catalogs, RSS feeds, Calibre's wireless transfer and KOReader sync. Three of them now have screens in the X4 Pro's stock firmware; on Boox and most other readers, KOReader covers OPDS, RSS and sync.
I've been through a lot of e-readers, and the X4 Pro is the one I recommend. At 72 g it fits in a pocket and still has physical page buttons, and its stock firmware now talks to book servers without flashing anything. If you want something else, my e-reader comparison table scores the rest.
A finished novel gets downloaded once. A serial, a course you're partway through, or notes on a subject you're still exploring keep getting longer. Sideloading a new file for every chapter scatters the book across your library and loses your place. You want one book in the library that you can refresh without losing your position.
My own reason: I use Gnothi, a tool I built, to generate an endless course on whatever I'm currently into, and I wrap a lot of it in fiction so it stays entertaining while I'm still learning. Those books grow every few days, which is why I went looking at these four. Since it's my product, weigh that part accordingly. A short note on it is at the end, and everything before it works with any server.
| What moves | Where the server runs | Use it when | |
|---|---|---|---|
| OPDS catalog | Book files, pulled by the reader | Anywhere on the internet or your network | You want to browse a library from the device and re-download books as they change |
| RSS feed | Individual items (articles, chapters, podcast episodes) | Anywhere on the internet | The source publishes item by item and your reading app takes feeds |
| Calibre wireless | Book files, pushed from your computer | Calibre on your computer, same Wi-Fi | Your library already lives in Calibre and you're at home |
| KOReader sync | Your reading position only, never files | A sync server, public or self-hosted | You read the same book on more than one device |
They combine. OPDS gets the book onto the device and KOReader sync carries your place between devices, while Calibre handles your own collection and RSS handles sources that publish one item at a time.
A recent update to Xteink's stock firmware added three screens: Calibre Transfer, KOReader Sync and an OPDS catalog list with Add catalog. I haven't found release notes or any public documentation for them, so the steps below come from my own unit, and a later update may rename or change them.
If you've flashed CrossPoint Reader, the open-source firmware that supports the X4 Pro, you already have an OPDS browser (up to 8 servers, with search and download), Calibre wireless and KOReader sync. svartling.net's CrossPoint walkthrough covers OPDS, Calibre-Web and KOReader sync on that firmware. I couldn't find one for the stock screens.
OPDS (Open Publication Distribution System) is a feed format for book catalogs. The reader asks the server for a list of books, shows it, and downloads the file you pick. Nothing is pushed to the device; you go and get it.
Servers that speak OPDS include Calibre's content server, Calibre-Web, COPS, Kavita and Komga, all of which you run on a computer or home server that stays on. Some public book sites publish one too. Standard Ebooks' full OPDS feed needs a Patrons Circle login, while its New Releases feed (https://standardebooks.org/feeds/opds/new-releases) is open. Project Gutenberg's OPDS feed gave me "connection failed" on the X4 Pro, with both the m. and www. addresses.
For a growing book, OPDS is the main tool: when new chapters land, you re-download the book from the catalog. On my X4 Pro, re-downloading the same file kept my reading position. I haven't yet confirmed that the position survives when the re-downloaded file is longer than the one I was reading, so test that with one book before you rely on it.
On the X4 Pro:
https://.The catalog then appears in the list, and you browse it and download books from there.
On KOReader (Kobo via NickelMenu, jailbroken Kindles, PocketBook, and Android devices including Boox): open the file browser, tap the magnifying-glass icon, choose OPDS, then tap the plus button at the top left to add a catalog name and URL, plus a username and password if needed. KOReader supports only HTTP basic authentication, so a server that demands digest auth won't log in. The KOReader OPDS page notes that a Calibre-Web address may need a trailing slash after opds.
A stock Kindle has no OPDS support; Send-to-Kindle is its route for getting files onto the device.
An RSS feed is a list of items, such as articles, chapters or podcast episodes (podcast feeds are RSS too). A reader that supports feeds checks for new items and fetches them, so RSS suits sources that publish item by item.
For a growing book, the catch is that you get a pile of separate items instead of one book with one bookmark. None of the three sync screens on the X4 Pro takes a feed.
KOReader has a News downloader that fetches feed items onto the device; you add the feed URL in its settings, then run Download news. On a Boox, which runs Android, you can install KOReader or any Android RSS reader. On a phone or computer, any podcast app or RSS reader works.
Calibre on your computer pushes books to the reader over your local network. Both have to be on the same Wi-Fi, and Calibre has to be running. There's nothing to host, and it stops working when you leave the house.
Use it when your library lives in Calibre. It doesn't help with a book that grows on someone else's server, unless you download the new version into Calibre first.
On the X4 Pro: open Calibre Transfer ("Send books from a computer"). Put the reader and the computer on the same Wi-Fi, then start the wireless device connection in Calibre. Tap Search to find your computer; there's also a Manual setup button. Books land in /Downloads/Calibre on the device.
On CrossPoint, the community advice is to send plain EPUB, not KEPUB.
KOReader sync stores your reading position on a server so another device can pick it up. It never moves files: each device needs its own copy of the book. The public server is https://sync.koreader.rocks, you can self-host koreader-sync-server instead, and Readest can use the same server.
Sync has to decide which file on each device counts as the same book. The KOReader progress sync page describes two methods. Binary, the default, matches on the file's exact contents, so only progress from an identical file syncs. Filename matches on the file name. CrossPoint's community recommends binary when syncing with KOReader. For a growing book, a re-downloaded, longer file has different contents, so on binary matching expect your progress to stop matching after a refresh; use filename matching where your reader has that option.
On the X4 Pro: open KOReader Sync ("Connect the same sync account, continue reading across devices"), then fill in Sync server (with https://), Username and Password. Tap Create account for a new account or Save account for an existing one. Account setup gave me trouble on the X4 Pro, so I can't yet confirm this screen works end to end.
In KOReader: the sync plugin appears only while a book is open. Use its Register/Login option to create the account on one device, then log in with the same details on the others.
Gnothi is an AI book maker I built. You give it a premise, pick fiction or nonfiction, and it writes the book one chapter at a time, on a schedule or when you ask, and narrates each chapter as audio. Chapters cost AI credits. Its OPDS catalog lists each of your series as a book.
The short version:
https://api.gnothiai.com/opds with the key as the password.API keys come with Gnothi's one-time unlock: any credit top-up turns them on for good. Without it, you can still download any series as an EPUB for free and sideload it. To try it without an account, add https://gnothiai.com/opds.xml, a free catalog of Gnothi's own books with no login. Gnothi's e-reader guide has the full X4 Pro and KOReader steps, the text RSS feed, and troubleshooting.