Iterate through multilevel boost tree

With my tree looking like this:

{
"Library":
{
    "L_ID": "1",
     "Book":
     {
         "B_ID": "1",
         "Title": "Moby Dick"
     }, 
     "Book":
     {
         "B_ID": "2",
         "Title": "Jurassic Park"
     }
},
"Library":
{
    "L_ID": "2",
     "Book":
     {
         "B_ID": "1",
         "Title": "Velocity"
     }, 
     "Book":
     {
        "B_ID": "2",
        "Title": "Creeper"
     }
}
}

What i am looking to do is iterate through the libraries. When i find the L_ID that i am looking for, iterate through the books until i find the B_ID i'm looking for. At that point, i'd like to access all the leaves in that section. I.e. looking for library 2, book 1, title Note: There's likely a better way than this.

boost::property_tree::ptree libraries = config_.get_child("Library");
for (const auto &lib : libraries)
{
   if (lib.second.get<uint16_6>("L_ID") == 2)
   { 
       //at this point, i know i'm the correct library...
       boost::property_tree::ptree books = lib.get_child("Book");
       for (const auto &book : books)
       {
            if (book.second.get<uint16_t>("B_ID") == 1)
            {
                 std::string mybook = book.second.get<std::string>("Title");
            }
        }
   }

I fail out as soon as i try looking into my first sub tree. What's going wrong here??