Trueman's Elementary Biology book I found says that the endoplasmic reticulum is absent in eggs, but no reason is given. Why is it absent?
I could find no indication that eggs are lacking an endoplasmic reticulum. On the contrary, I could find papers written on the ER in eggs of several species.
However, as can be read in the sea urchin paper, the ER changes after fertilization:
We interpret these observations to indicate that in the first few minutes after fertilization, the egg's ER is fragmented
For other species, I could only find evidence that the ER reorganizes after fertilization because of changed calcium signaling needs