See the Bridge pattern in [6].

The declaration file:

// If you are not planning on applying dynamic_cast<> to 
// the public interface base class, then it does not need to be virtual
struct Book : boost::impl_ptr<Book>::shared
{
   Book(args);

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

struct ThickBook : Book
{
   ThickBook(args);
};

struct ThickestBook : ThickBook
{
   ThickestBook(args);
};

The implementation file:

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

   virtual ~implementation() {...}
   ...
};

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

struct ThickestBookImpl : ThickBookImpl
{
   ThickestBookImpl(args) : ThickBookImpl(args) {...}
   ...
};

ThickBook:: ThickBook(args) : Book(boost::impl_ptr<Book>::null())
{
   emplace<ThickBookImpl>(args);
   ...
}

ThickestBook::ThickestBook(args) : ThickBook(boost::impl_ptr<ThickBook>::null())
{
   emplace<ThickestBookImpl>(args);
   ...
}