target = object_position
pos.x = zoom * sin(phi) * cos(theta);
pos.y = zoom * cos(phi);
pos.z = zoom * sin(phi) * sin(theta);
pos += target;
gluLookAt(pos, target, vec3(0, 1, 0)); // up vector is fixed...
The code above creates a 'camera' that is looking at the object centre and can rotate around (using spherical coords).