boost::hash is implemented by calling the function hash_value. The namespace isn't specified so that it can detect overloads via argument dependant lookup. So if there is a free function hash_value in the same namespace as a custom type, it will get called.

If you have a structure library::book, where each book is uniquely defined by it's member id:

namespace library
{
    struct book
    {
        int id;
        std::string author;
        std::string title;

        // ....
    };

    bool operator==(book const& a, book const& b)
    {
        return a.id == b.id;
    }
}

Then all you would need to do is write the function library::hash_value:

namespace library
{
    std::size_t hash_value(book const& b)
    {
        boost::hash<int> hasher;
        return hasher(b.id);
    }
}

And you can now use boost::hash with book:

library::book knife(3458, "Zane Grey", "The Hash Knife Outfit");
library::book dandelion(1354, "Paul J. Shanley",
    "Hash & Dandelion Greens");boost::hash<library::book> book_hasher;
std::size_t knife_hash_value = book_hasher(knife);

// If std::unordered_set is available:
std::unordered_set<library::book, boost::hash<library::book> > books;
books.insert(knife);
books.insert(library::book(2443, "Lindgren, Torgny", "Hash"));
books.insert(library::book(1953, "Snyder, Bernadette M.",
    "Heavenly Hash: A Tasty Mix of a Mother's Meditations"));

assert(books.find(knife) != books.end());
assert(books.find(dandelion) == books.end());

The full example can be found in: /libs/functional/hash/examples/books.hpp and /libs/functional/hash/examples/books.cpp.