diff --git a/src/sidebar.js b/src/sidebar.js
index 5d4cdf3..d7c23d6 100644
--- a/src/sidebar.js
+++ b/src/sidebar.js
@@ -1,4 +1,9 @@
 var concurrency = 5
+var retry_delay = 15/* seconds */
+
+function sleep(delay) {
+    return new Promise((resolve) => setTimeout(resolve, delay*1000));
+}
 
 function getCurrentTab() {
     console.debug("Querying active tab.");
@@ -65,7 +70,8 @@ class Book{
             .catch((error)=>{
                 if (!retry){
                     console.warn(`Failed ${url} - ${error.message}`)
-                    return this.downloadResource(url, true)
+                    return sleep(retry_delay)
+                            .then(()=>{return this.downloadResource(url, true)})
                 } else {
                     console.error(`Giving up on ${url} - ${error.message}`)
                     throw `Failed second try on ${url} - ${error.message}`
@@ -76,7 +82,8 @@ class Book{
                     return res
                 } else if (!retry){
                     console.warn(`Failed ${url} - ${res.status} - ${res.statusText}`)
-                    return this.downloadResource(url, true)
+                    return sleep(retry_delay)
+                            .then(()=>{return this.downloadResource(url, true)})
                 } else {
                     console.error(`Giving up on ${url} - ${res.status} - ${res.statusText}`)
                     throw `Failed second try on ${url} - ${res.status} - ${res.statusText}`
@@ -174,22 +181,26 @@ class Book{
 
     downloadContent(){
         return Promise.map(Object.keys(this.book_files), (url) => {
-                return this.downloadAndReport(url, this.book_files[url].filename)
-                    .then((res) => {
-                        if (res.ok){
-                            this.book_files[url].headers = res.headers
-                            this.book_files[url].mime = res.headers.get('Content-Type')
-                            return res.arrayBuffer().then((arrBuffer)=>{
-                                this.book_files[url].body = arrBuffer
-                            })
-                        }
-                    })
-                    .catch((error)=>{
-                        throw `Failed to download book content: "${error}"`
-                    })
+                return this.downloadContentFile(url)
             },{concurrency: concurrency})
     }
 
+    downloadContentFile(url){
+        return this.downloadAndReport(url, this.book_files[url].filename)
+        .then((res) => {
+            if (res.ok){
+                this.book_files[url].headers = res.headers
+                this.book_files[url].mime = res.headers.get('Content-Type')
+                return res.arrayBuffer().then((arrBuffer)=>{
+                    this.book_files[url].body = arrBuffer
+               })
+            }
+        })
+        .catch((error)=>{
+            throw `Failed to download book content: "${error}"`
+        })
+    }
+
     insertBookFile(url, filename){
         this.book_files[url] = {
             url: url,