Not sure whats on page 346 because I do not have the book myself. But I'm trying to help.
If it is about loading x models, the normals are already present in the x file, only tangents are recalculated in my TangentVertexProcessor. It is however possible to generate normals if they are not present with the ComputeNormals method of the ModelMesh class there (same way as ComputeTangents is used).
Later in the book normals are calculated for own geometry like the road, the landscape, etc. In most cases, especially in unit tests, the normals are pre-computed and often just point straight up (e.g. for a ground plane). In case of the landscape normal computation, which is probably the most advanced and complex, it works by taking several surrounding landscape points and calculating the vectors from the current landscape point to them and then taking the cross product of them and interpolating several cross products together.
That code and more comments can be found in the Landscape.cs file (line 802-830):
// Step 1: Calculate position
int index = x + y * GridWidth;
Vector3 pos = CalcLandscapePos(x, y, heights);//texData);
mapHeights[x, y] = pos.Z;
vertices[index].pos = pos;
// Step 2: Calculate all edge vectors (for normals and tangents)
// This involves quite complicated optimizations and mathematics,
// hard to explain with just a comment. Read my book :D
Vector3 edge1 = pos - CalcLandscapePos(x, y + 1, heights);//texData);
Vector3 edge2 = pos - CalcLandscapePos(x + 1, y, heights);//texData);
Vector3 edge3 = pos - CalcLandscapePos(x - 1, y + 1, heights);//texData);
Vector3 edge4 = pos - CalcLandscapePos(x + 1, y + 1, heights);//texData);
Vector3 edge5 = pos - CalcLandscapePos(x - 1, y - 1, heights);//texData);
// Step 3: Calculate normal based on the edges (interpolate
// from 3 cross products we build from our edges).
vertices[index].normal = Vector3.Normalize(
Vector3.Cross(edge2, edge1) +
Vector3.Cross(edge4, edge3) +
More code is also in the Landscape constructor, which smoothes the normals again to make the lighting appear more settle throughout the landscape.
Hope this helps.