diff --git a/README.md b/README.md
index 395d2fc..a5096cb 100644
--- a/README.md
+++ b/README.md
@@ -81,8 +81,12 @@ $ book sm --help
 
 **Notes**： 
 * The article title is taken from `title` property in the articles front-matter. If this property is not available, the articles filename will be used as title for the summary. 
-* The option `-s` or `--sortedBy` can not be given `-` as argument, because commander.js will parse it an option. But you can set it in `book.json` as follows.
-
+* The option `-s` or `--sortedBy` can not be given `-` as argument, because commander.js will parse it an option. But you can set it in `book.json` as follows.  
+* set up the sortedBy and if there are any summaries missing the order, look at the example below and follow,  
+for example, you have summaries like this `01-elementry-school, 02-middle-school, 03-university, ...`  
+you realized high school was missing, then You can make correct order in the following way  
+eg. `01-elementry-school, 02-middle-school, 02a-high-school, 03-university, ...`  
+not `01-elementry-school, 02-middle-school, 03-high-school, 04-university, ...`
 
 2> Create a `book.json` in the book`s root folder
 
diff --git a/lib/files.js b/lib/files.js
index 41b4675..414728c 100644
--- a/lib/files.js
+++ b/lib/files.js
@@ -1,48 +1,43 @@
 var _ = require('lodash');
 var fs = require('fs');
 var path = require('path');
-var async = require('async');
 var color = require('bash-color');
 var fm = require('front-matter');
 
+// separate for test
+var sort = function(current, next, sortedBy) {
+  if (current.isDirectory && !next.isDirectory) return -1;
+  if (!current.isDirectory && next.isDirectory) return 1;
+  // Sorted if given current sorted hyphen, for example: `-` or `_`
+  if (sortedBy) {
+    var pattern = "(^\\d*)" + sortedBy;
+    var reg = new RegExp(pattern);
+    if(current.name.match(reg) && next.name.match(reg)){
+      var currentNum = current.name.match(reg)[1];
+      var nextNum = next.name.match(reg)[1];
+      return currentNum - nextNum;
+    }
+  }
+  return current.name.localeCompare(next.name);
+};
+
 // Use a loop to read all files
 function ReadFile(filePath, filesJson, sortedBy) {
   var files;
 
-  try {
-    // Synchronous readdir
-    files = fs.readdirSync(filePath)
-      // sort the files: directories first, afterwards files
-      .map(function(v) {
-        var stat = fs.statSync(path.resolve(filePath, v));
-        return {
-          name: v,
-          isDirectory: stat.isDirectory()
-        };
-      })
-      .sort(function(a, b) {
-        if (a.isDirectory && !b.isDirectory) return -1;
-        if (!a.isDirectory && b.isDirectory) return 1;
-        // Sorted if given a sorted hyphen, for example: `-` or `_`
-        if (sortedBy) {
-            var pattern = "(^[\\da-zA-Z]*)" + sortedBy;
-            var reg = RegExp(pattern);
-            if(a.name.match(reg) && b.name.match(reg)){
-              var aNum = a.name.match(reg)[1];
-              var bNum = b.name.match(reg)[1];
-              return aNum - bNum;
-            }
-        }
-        return a.name.localeCompare(b.name);
-      })
-      .map(function(v) {
-        return v.name;
-      });
+  function getFrontMatterTitle(newpath)
+  {
+    // default to use filename
+    var title = path.parse(newpath).name;
+    var content = fs.readFileSync(newpath,'utf8');
 
-    files.forEach(walk);
-  } catch (error) {
-    filesJson = null; //fixme
-    console.log(color.red(error.message));
+    if (!fm.test(content)) return title;  // skip if no front matter
+
+    var frontMatter = fm(content);
+    if (typeof frontMatter.attributes.title === "undefined") return title;      // skip if no 'title' attributes
+    // todo: set 'title' via config
+
+    return frontMatter.attributes.title;
   }
 
   function walk(file) {
@@ -51,7 +46,7 @@ function ReadFile(filePath, filesJson, sortedBy) {
 
     if (state.isDirectory()) {
       filesJson[file] = {};
-      ReadFile(newpath, filesJson[file], sortedBy);
+      new ReadFile(newpath, filesJson[file], sortedBy);
       // filter empty directories
       if (Object.keys(filesJson[file]).length < 1) {
         delete filesJson[file];
@@ -66,20 +61,32 @@ function ReadFile(filePath, filesJson, sortedBy) {
     }
   }
 
-  function getFrontMatterTitle(newpath)
-  {
-    // default to use filename
-    var title = path.parse(newpath).name;
-    var content = fs.readFileSync(newpath,'utf8');
-
-    if (!fm.test(content)) return title;  // skip if no front matter
-
-    var frontMatter = fm(content);
-    if (typeof frontMatter.attributes.title === "undefined") return title;      // skip if no 'title' attributes
-    // todo: set 'title' via config
+  try {
+    // Synchronous readdir
+    files = fs.readdirSync(filePath)
+      // sort the files: directories first, afterwards files
+      .map(function(v) {
+        var stat = fs.statSync(path.resolve(filePath, v));
+        return {
+          name: v,
+          isDirectory: stat.isDirectory()
+        };
+      })
+      .sort(function(current, next) {
+        return sort(current, next, sortedBy);
+      })
+      .map(function(v) {
+        return v.name;
+      });
 
-    return frontMatter.attributes.title;
+    files.forEach(walk);
+  } catch (error) {
+    filesJson = null; //fixme
+    console.log(color.red(error.message));
   }
 }
 
-module.exports = ReadFile;
+module.exports = {
+  readFile: ReadFile,
+  sort: sort
+};
diff --git a/lib/summary/index.js b/lib/summary/index.js
index 0950ab7..d2fec5e 100644
--- a/lib/summary/index.js
+++ b/lib/summary/index.js
@@ -6,7 +6,7 @@ var async = require('async');
 
 var bookJson = require('../bookJson');
 var utils = require('../utils');
-var readFile = require('../files');
+var readFile = require('../files').readFile;
 
 /**
  * Get a summary from a fold such as `/path/to/your/book` or `../book`
diff --git a/test/files.test.js b/test/files.test.js
new file mode 100644
index 0000000..c05e6f8
--- /dev/null
+++ b/test/files.test.js
@@ -0,0 +1,29 @@
+var should = require('should');
+var sort = require('../lib/files').sort;
+
+describe('files.js', function () {
+  var DirClass = function(name) {
+    this.name = name;
+    this.isDirectory = true;
+  };
+
+  it('test sort with alphabet', function () {
+    var order = [new DirClass('01-a'), new DirClass('02-a'), new DirClass('03a-a'), new DirClass('03-a'), new DirClass('04-a')];
+    var sorted = order.sort(function(current, next) {
+      return sort(current, next, '-');
+    });
+
+    should.deepEqual(sorted,
+      [new DirClass('01-a'), new DirClass('02-a'), new DirClass('03-a'), new DirClass('03a-a'), new DirClass('04-a')]);
+  });
+
+  it('test sort only digit', function () {
+    var order = [new DirClass('01-a'), new DirClass('2-a'), new DirClass('7-a'), new DirClass('10-a'), new DirClass('04-a')];
+    var sorted = order.sort(function(current, next) {
+      return sort(current, next, '-');
+    });
+
+    should.deepEqual(sorted,
+      [new DirClass('01-a'), new DirClass('2-a'), new DirClass('04-a'), new DirClass('7-a'), new DirClass('10-a')]);
+  });
+});