For details and explanations see the Non-Virtual Interface idiom in [11] and Rule #39 in [12].

struct Book : boost::impl_ptr<Book>::shared // Not virtual
{
   Book(args);

   string const&  title() const;
   string const& author() const;
};

template<> struct boost::impl_ptr<Book>::implementation
{
   implementation(args) {...}

   virtual ~implementation() {...}
}

struct Implementation1 : boost::impl_ptr<Book>::implementation
{
   Implementation1() : implementation(args) { ... }
};

struct Implementation2 : boost::impl_ptr<Book>::implementation
{
   Implementation2() : implementation(args) { ... }
};

Book::Book(args) : impl_ptr_type(nullptr)
{
   some_condition
       ? impl_ptr_type::emplace<Implementation1>(args)
       : impl_ptr_type::emplace<Implementation2>(args);
}