diff --git a/src/epub.js b/src/epub.js
index 7984b9d..75b2d95 100644
--- a/src/epub.js
+++ b/src/epub.js
@@ -8,40 +8,6 @@ var xmlSerializer = new XMLSerializer();
 var domParser = new DOMParser();
 var generatedBufferType = 'blob'
 
-function Utf8ArrayToStr(array) {
-    var out, i, len, c;
-    var char2, char3;
-
-    out = "";
-    len = array.length;
-    i = 0;
-    while(i < len) {
-    c = array[i++];
-    switch(c >> 4)
-    {
-      case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:
-        // 0xxxxxxx
-        out += String.fromCharCode(c);
-        break;
-      case 12: case 13:
-        // 110x xxxx   10xx xxxx
-        char2 = array[i++];
-        out += String.fromCharCode(((c & 0x1F) << 6) | (char2 & 0x3F));
-        break;
-      case 14:
-        // 1110 xxxx  10xx xxxx  10xx xxxx
-        char2 = array[i++];
-        char3 = array[i++];
-        out += String.fromCharCode(((c & 0x0F) << 12) |
-                       ((char2 & 0x3F) << 6) |
-                       ((char3 & 0x3F) << 0));
-        break;
-    }
-    }
-
-    return out;
-}
-
 class EpubXhtml{
     constructor(filename) {
         this.filename = filename
diff --git a/src/sidebar.js b/src/sidebar.js
index ab9a21e..42ef4d6 100644
--- a/src/sidebar.js
+++ b/src/sidebar.js
@@ -1,3 +1,5 @@
+var concurrency = 5
+
 function getCurrentTab() {
     console.debug("Querying active tab.");
     var queryInfo = {
@@ -38,7 +40,7 @@ function extractBookId(url){
 
 class Book{
 
-    constructor(book_id, epub) {
+    constructor(book_id, page) {
         this.book_id = book_id
         this.raw_book = {}
         this.chapter_list = []
@@ -47,17 +49,24 @@ class Book{
         this.book_info = null
         this.book_toc = null
         this.book_flattoc = null
+        this.page = page
     }
 
     downloadResource(url){
         console.info(`Downloading ${url}`)
         return fetch(url, {
-            credentials: 'include',
-            mode: "no-cors" // no-cors, cors, *same-origin
-        }).then((res) => {
-            // console.log(`Downloaded.`)
-            return res;
-        }, onError)
+                credentials: 'include',
+                mode: "no-cors" // no-cors, cors, *same-origin
+            })
+            .catch((error)=>{
+                console.info(`Failed ${url} - ${error.message}`)
+                console.info(`Retry ${url}`)
+                // If failed to download, try second time.
+                return fetch(url, {
+                    credentials: 'include',
+                    mode: "no-cors" // no-cors, cors, *same-origin
+                })
+            })
     }
 
     downloadJson(url){
@@ -95,13 +104,19 @@ class Book{
             }))
         downloads.push(
             Promise.map(this.book_info.chapters, (chapter) => {
-                renderProgress(`${chapter}`)
+                let item_id = this.page.renderProgress(`${chapter}`)
                 return this.downloadJson(chapter)
                         .then((json) => {
                             this.chapter_info[chapter] = json
                             return this.extractChapterAssets(json)
                         })
-            },{concurrency: 10}))
+                        .then(()=>{
+                            this.page.renderDone(item_id)
+                        })
+                        .catch((error)=>{
+                            this.page.renderFail(item_id, error)
+                        })
+            },{concurrency: concurrency}))
 
         return Promise.all(downloads)
     }
@@ -131,7 +146,7 @@ class Book{
 
     downloadContent(){
         return Promise.map(Object.keys(this.book_files), (url) => {
-            renderProgress(`${this.book_files[url].filename}`)
+            let item_id = this.page.renderProgress(`${this.book_files[url].filename}`)
             return this.downloadResource(url)
                     .then((res) => {
                         if (res.ok){
@@ -141,8 +156,15 @@ class Book{
                                 this.book_files[url].body = arrBuffer
                             })
                         }
-            })
-        },{concurrency: 10})
+                    })
+                    .then((arrBuffer)=>{
+                        this.page.renderDone(item_id)
+                        return arrBuffer
+                    })
+                    .catch((error)=>{
+                        this.page.renderFail(item_id, error)
+                    })
+        },{concurrency: concurrency})
     }
 
     insertBookFile(url, filename){
@@ -163,6 +185,7 @@ class SidebarPage{
         $('#error-message').hide();
         $('#book-info').hide();
         $("#book-file-list").empty();
+        this.id = 1
     }
 
     renderInfo(book){
@@ -187,6 +210,31 @@ class SidebarPage{
         }
         $('#loading').hide();
     }
+
+    renderProgress(txt){
+        let id = `item_${this.id}`
+        this.id++
+        // Add chapters to UI
+        var progress_dom = $("<li></li>")
+                    .addClass("list-group-item")
+                    .attr('id', id)
+                    .html(txt)
+        $("#book-file-list").prepend(progress_dom);
+        return id
+    }
+
+    renderDone(id){
+        $( '#'+id )
+            .css( "color", "green" )
+            .append(' - OK');
+    }
+
+    renderFail(id, reason){
+        $( '#'+id )
+            .css( "color", "red" )
+            .append(' - Fail')
+            .attr('title',reason.message);
+    }
 }
 
 function onError(error) {
@@ -197,13 +245,6 @@ function onError(error) {
     $('#book-info').hide();
 }
 
-function renderProgress(txt){
-    // Add chapters to UI
-    var progress_dom = $("<li></li>")
-                .addClass("list-group-item")
-                .html(txt)
-    $("#book-file-list").prepend(progress_dom);
-}
 
 
 function fillMetadata(epub, book)
@@ -322,7 +363,7 @@ function onDownloadBookClicked(){
     .then(extractBookId, onError)
     .then((book_id) => {
         epub = new EpubWriter();
-        book = new Book(book_id);
+        book = new Book(book_id, page);
         book.downloadBookInfo()
         .then(() => { page.renderInfo(book); })
         // .then(() => { return book.downloadChapterList(); })
@@ -338,7 +379,7 @@ function onDownloadBookClicked(){
                 + title.replace(/[^a-z0-9]/gi, '_').toLowerCase()
                 + ".epub"
             console.log(`Zip file name ${filename}`)
-            renderProgress(`Saved to ${filename}`)
+            page.renderProgress(`Saved to ${filename}`)
             let url = window.URL.createObjectURL(file)
             return browser.downloads.download({ "filename" : filename, url : url})
         })