Standard Free energy of hydrolysis of phosphoenolpyruvate (PEP)
An answer to this is given in the online version of Berg et al.


  Why does phosphoenolpyruvate have such a high phosphoryl-transfer potential? The phosphoryl group traps the molecule in its unstable enol form. When the phosphoryl group has been donated to ATP, the enol undergoes a conversion into the more stable ketone—namely, pyruvate.





  Thus, the high phosphoryl-transfer potential of phosphoenolpyruvate arises primarily from the large driving force of the subsequent enol-ketone conversion


Note that Berg et al. use the term “high phosphoryl-transfer potential” or “high phosphoryl group-transfer potential”, as explained in sections 14.1.4 and 14.1.5. These sections are worth reading, and this terminology is worth following.