public static class GlobalDB
{
    EntityTables DatabaseInstance; //imaginary singleton
}

public class EntityTables
{
    //imagine this is a singleton

    public IQueryable<User> UserTable
    {
        get
        {
            DB.instance.GetAllUsers();
        }
    }

}

public class User
{
    public int userID {get; set;}
    List<UserBook> UserBooks
    {
        get
        {
            DB.instance.GetUserBooksByUserID(userID);
        }
    }
}

public class anotherClassSomewhere
{
    User currentUser =
    GlobalDB.DatabaseInstance.UserTable.Where(u => u.userID == 4).SingleOrDefault();
}