I've read that book; it's a good layman survey. Robinson mentions that one of the experts in epigraphy gets hundreds of supposed solutions to the Phaistos disk sent to him every year.
Those supposed decipherments highlight the problem of decipherment of an unknown language. How would you know if the decipherment is accurate? So far, there are no claimed decipherments of Linear A or the Phaistos disk or Rongo-Rongo that are taken seriously "in the field." But probably a different word than decipherment needs to be used if the language is not known, and the solution cannot be verified.
There were of course comical early "decipherments" of glyphs that have since been largely cracked, such as Egyptian and Mayan.
"How would you know if the decipherment is accurate?"
The Ventris decipherment was rejected (actually, before it was made) by Arthur Evans' followers. Evans had insisted that the Minoans hadn't been Greek speakers, and that their written language would bear that out. Blegen went to Pylos hoping to find an archive and found one. Evans' followers immediately claimed the tablets had originated on Crete, but had been carried off by the conquering Greeks.
Then Blegen et applied Ventris' decipherment to the Pylos tablets. :')
http://www.freerepublic.com/focus/f-chat/1202723/posts?page=12#12