There is a small error in the implementation of Camera::lookAt in the opengl_series project. Please fix it by replacing "radians" with "degrees" as shown below:

void Camera::lookAt(glm::vec3 position) {
    assert(position != _position);
    glm::vec3 direction = glm::normalize(position - _position);
    _verticalAngle = glm::radiansdegrees(asinf(-direction.y));
    _horizontalAngle = -glm::radiansdegrees(atan2f(-direction.x, -direction.z));
    normalizeAngles();
}