I am sure you've probably noticed, that Pentangeli, who is supposed to be a close, trusted, old family friend, a friend of Vito's from way back, is not mentioned anywhere in the book. Just like he was nowhere to be seen in GF I.

Pentangeli was a character written in at the last minute when Paramount and FFC couldn't get Richard Castellano, Clemenza in the first movie, to sign on for GF II. Castellano had been, somewhat amazingly, the highest-paid actor in GF I (which reportedly did not sit well with the rest of the cast). In the meantime, of course largely because of the success of GF I, Pacino, Keaton and Duvall had become A-list stars.

Castellano, reportedly, also wanted to be paid the most for GF II. When he didn't get it, he walked (although not before appearing for one moment in GF II as one of the street spectators at the St. Rocco's Day parade, as Vito is tracking down Fanucci).

So FFC found himself with shooting deadlines for GF coming up fast, and no Castellano. Instead of just recasting the role of Clemenza, he created the replacement character of Pentangeli, who was supposed to have been a protege/pal of Clemenza.

The Pentangeli character was very similar to Clemenza, so most of the script lines already written for Clemenza were also used for Pentangeli. Like Clemenza, he was supposedly an old pal of Vito from way back. Like Clemenza he was hot to break off and form his own family and control his own territory, and thus be a threat to the Corleones.

All they had to do was put in a couple lines explaining that "old Clemenza" had died, and making sure everybody called Michael F. Gazza "Pentangeli," and everything was OK.