From https://once.com/writebook

app/javascript/helpers/reading_progress_helpers.js

@@ -0,0 +1,16 @@
+import { setCookie, readCookie } from "helpers/cookie_helpers"
+
+export function getReadingProgress(bookId) {
+  const progress = readCookie(`reading_progress_${bookId}`)
+
+  if (progress) {
+    const [ leafId, lastReadParagraph ] = progress.split("/")
+    return [ parseInt(leafId), parseInt(lastReadParagraph) || 0 ]
+  } else {
+    return [ undefined, 0 ]
+  }
+}
+
+export function storeReadingProgress(bookId, leafId, lastReadParagraphIndex) {
+  setCookie(`reading_progress_${bookId}`, `${leafId}/${lastReadParagraphIndex}`)
+}