Fix the bad Rakefile.

Rakefile

@@ -2,25 +2,27 @@ task :default => "book"
 
 task book: [:epub, :pdf, :clean]
 
-if File.exists?('working')
-  sh "rm -rf ./working"
+WORKING_DIR = './working'
+RELEASE_DIR = './release'
+
+file init: FileList['./*.md'] do |task|
+  mkdir WORKING_DIR unless File.exists?(WORKING_DIR)
+  cp task.prerequisites, WORKING_DIR
+  mv "#{WORKING_DIR}/README.md", "#{WORKING_DIR}/000.README.md"
 end
-sh "mkdir ./working"
-sh "cp *.md working/"
-sh "mv working/README.md working/000.README.md"
 
-file html: Dir.glob('./working/*.md') do |task|
-  sh "script/export_html.rb #{task.prerequisites.join(' ')} > release/process_book.html"
+task html: %w[init] do
+  sh "script/export_html.rb #{FileList["./#{WORKING_DIR}/*.md"].join(' ')} > #{RELEASE_DIR}/process_book.html"
 end
 
 task epub: %w[html] do
-  sh "ebook-convert release/process_book.html release/process_book.epub --no-default-epub-cover"
+  sh "ebook-convert #{RELEASE_DIR}/process_book.html #{RELEASE_DIR}/process_book.epub --no-default-epub-cover"
 end
 
 task pdf: %w[html] do
-  sh "wkhtmltopdf release/process_book.html release/process_book.pdf --encoding utf8"
+  sh "wkhtmltopdf #{RELEASE_DIR}/process_book.html #{RELEASE_DIR}/process_book.pdf --encoding utf8"
 end
 
 task :clean do
-  sh "rm -rf working"
+  sh "rm -rf #{WORKING_DIR}"
 end