Add external library to vs code c++ linux - c++

Question

I'm trying to use libxl in my c++ project on visual studio code. This is my directory map:
├── ken.code-workspace
└── src
├── include
│ ├── AutoFilterA.h
│ ├── AutoFilterW.h
│ ├── BookA.h
│ ├── BookW.h
│ ├── enum.h
│ ├── FilterColumnA.h
│ ├── FilterColumnW.h
│ ├── FontA.h
│ ├── FontW.h
│ ├── FormatA.h
│ ├── FormatW.h
│ ├── FormControlA.h
│ ├── FormControlW.h
│ ├── handle.h
│ ├── libxl.h
│ ├── RichStringA.h
│ ├── RichStringW.h
│ ├── setup.h
│ ├── SheetA.h
│ └── SheetW.h
├── ken.cpp
└── lib
└── libxl.so
And this is my ken.cpp:
#include <iostream>
#include <vector>
#include <string>
#include "include/libxl.h"
using namespace std;
using namespace libxl;
int main()
{
cout << "Hello babe :)" << endl;
Book* book = xlCreateBook();
}
But I get these errors while trying to compile:
/run/media/mahdi/Mahdi/Projects/ken/src/ken.cpp:7:17: error: ‘libxl’ is not a namespace-name
7 | using namespace libxl;
| ^~~~~
/run/media/mahdi/Mahdi/Projects/ken/src/ken.cpp: In function ‘int main()’:
/run/media/mahdi/Mahdi/Projects/ken/src/ken.cpp:12:5: error: ‘Book’ was not declared in this scope
12 | Book* book = xlCreateBook();
| ^~~~
/run/media/mahdi/Mahdi/Projects/ken/src/ken.cpp:12:11: error: ‘book’ was not declared in this scope; did you mean ‘bool’?
12 | Book* book = xlCreateBook();
| ^~~~
| bool
I've never added an external library to visual studio code project before. Can I know what I'm doing wrong?