All of the usability methods for document reading are on the document::view class.  However, as a usability improvement, it would be nice to provide operator[] on the document::value class, which would delegate the real work to document::view under the hood:

auto doc = make_document(kvp("a", 1));

// currently
auto a = doc.view()["a"].get_value(); 

// also currently
auto doc_view = doc.view();
auto a = doc_view["a"].get_value();

// proposed
auto a = doc["a"].get_value();