Every musician wants to jump to syncopation and irregular meters. The first 20 exercises of Pozzoli seem "too easy." Do them anyway. They establish the relationship between your voice and the pulse.

Speaking rhythm without a metronome is worthless. The human voice naturally accelerates. The metronome is your judge.

If you are looking for the classic music theory manual Pozzoli: Solfeos Hablados y Cantados