Sorry you've had to chase down things. I feel your pain... The Cake core has been adjusted since the book's publication, but that's a good thing; it means that Cake will be all that much better once the official 1.2 release makes it out of beta, etc. We wrestled with some of these questions during production, wondering how best to write the book knowing that anything could change in the near future. But I opted to get the book out there. At the time, there was no Cake book in English, and the Packt Book came out days before mine (I believe it was probably on their minds since my book was announced first, and they made no mention until theirs hit bookshelves, like it was their strategy to proclaim it as the first...). Documentation was sorely needed, and that was first priority.
Since then, the book.cake.org site has improved greatly, and another Cake book has come out ("Practical CakePHP Projects"). Sorry for the long story, but I hope it does at least put some of my writing in this book in perspective.
That said--more than you wanted, I know--I'm glad to see that you, and others, have come to this forum. I apologize for not keeping up with it as much; grad studies have me swamped as well as some pressing web assignments. But it looks like many others are here to help me in my attempts to keep up with the errata, for which I'm grateful.
OK--the question in this thread...
You shouldn't have to specify too much in the Ajax::form() function. I'm pretty sure
$ajax->form('Comment','post',array('url'=>array('controller'=>'comments','action'=>'add'),'update'=>'comments'));
should work.