Rid the kindle "documents" directory of unnecessary *.sdr record files. Optional parameter: full path of the documents directory.

clean_up_kinde_documents_dir.rb

require 'FileUtils'
root = ARGV[0] || "/Volumes/Kindle/documents"

all_files = Dir[root + "/*"]
rec_files = all_files.find_all{ |f| f.end_with?(".sdr") }

# doc_types = ['txt', 'pdf', 'mobi', 'azw']
doc_files = all_files.find_all do |f|
  f.end_with?("mobi") ||
  f.end_with?("pdf") ||
  f.end_with?("txt") ||
  f.end_with?("azw")
end
doc_files_string = doc_files.join(" ").gsub(/\(|\)/, '')

rec_to_del = rec_files.reject do |f|
  # have the same name
  book_name = f.gsub(/\.sdr|\(|\)/, '').gsub(Regexp.new(root + "/"), '')
  short_name = book_name.slice(0, 20)
  book_exist = doc_files_string.match(short_name)

  if !book_exist
    print 'Deleted record: ', f, "\n"
  end

  book_exist
end

rec_to_del.each do |f|
  FileUtils.rm_rf(f)
end

Use it with:

$ ruby clean_up_kinde_documents_dir.rb /full/path/to/documents/folder