The main thing that you want to obtain from any book that you read, is the information from it that MATTERS to YOU. So, in your asking if it is absolutely necessary for you to comprehend and obtain all the information - that all depends.
First off, how much time are you alotting in between PhotoReading and activation? Sometimes, incubation time can make all the difference in understanding the topic at hand.
If you want all of the information, then previewing, PhotoReading, post-previewing and then Rapid Reading would be my choice. Once you start with a base, you can work your way up.
The PhotoReading Whole Mind System is all about layers. Sort of like computer programming. We began with binary. 00100011011000.
Then - we worked our way up and got to C++, and all that great stuff.
Technology and knowledge today could not have gotten here without the base knowledge.
But hey! That's another topic entirely. Basically my point is - incubation time is probably the key. I could be wrong on this one. But as far as what I've seen in the past, I'll blaze through a book and it wouldn't gel. But if I would have waited a day - it woulda made sense. So I try it again, and WHALA!
So anyways, let us know how much incubation time you're allowing. It's probably something else, and I'm probably way off, but whatever. Let us know. =P
-youngprer