leads to http://www.web2py.com/book/default/04#markmin_URL
which returns invalid function (default/04)
instead of http://www.web2py.com/books/default/chapter/29/04/the-core#URL
When you are viewing chapter 4 text (ie from here: )http://web2py.com/books/default/chapter/29/05/the-views
the link works.
The erroneous link you see looks to me like you accessed it via searching. There is a patch to fix this but I think Massimo needs to reload the app, the changes haven't gone live. I'll email him about it.
Can be closed can't reproduce with the actual online book. All the refered links are working as they should... Mean you never get redirected over : http://www.web2py.com/book/default/04#markmin_URL
But over : http://web2py.com/books/default/chapter/29/04#markmin_URL which brings you on the proper content...