[chaley]

You should look at "template program mode" in http://calibre-ebook.com/user_manual/template_lang.html. That mode allows you to write more arbitrary programs. For example, your original program could be written as

{#gr_2read:'test($,"To Read",strcat(field("author_sort"),"/",field("series")))'}/{title} - {authors}

or

{#gr_2read:'test($,"To Read",template("[[author_sort]]/[[series]]"))'}/{title} - {authors}

Use "general program mode" for even more flexibility.

program:
  strcat(
    test(field('#gr_2read'), "To Read", template("{author_sort}/{series}")),
    template('/{title} - {authors}')
  )