From the Camera::Update methods (I mean those from the CD-ROM) it can be seen that

a) mvView is set to a world position which is targeted by the camera (mvTargetPosition, vGoadViewpoint, or similar), and

b) mvFacing is set to the direction to that target (normalized difference from mvPosition to mvView).

Later on in Camera::Look() one can see that mvVew is directly used as center argument for gluLookAt, which again means a target position (see e.g. manual page for gluLookAt).

The difference between 2 positions is calculated by subtracting the start position from the end position; TiagoCosta has mentioned it. However, the view direction of an unrotated camera is convention. It may look along any positive or negative axis! Since you adapt the source code for Ogre, you have to pick the convention as is used by Ogre, and compare that with the convention used in the article's source code; perhaps you have to adapt this part, too.