No, it's not because it breaks the 3S rule--having 3 S sounds all together.
That book is wrong. There's no such rule that relates to orthography in any of the main style books--Chicago Manual of Style, AP Stylesheet, MLA, etc.
In any case, the possessive "s" is pronounced with a "z" sound there, so there aren't three s sounds in a row.
On edit: I Googled a bit, and found all kinds of rules about this, including one site (Dictionary.com) that had an incredibly elaborate set of rules that included "If there is already a sibilant in the word, don't add another s:
Kansas', not
Kansas's."
I guess this is a less settled issue than I had thought--you can find many authorities that say the possessive of "boss" should be "boss's", and many authorities that say the possessive of "boss" should be "boss'".
So, Doug, whichever you do, some people will think it's right and other people will think it's wrong.