intheclouds:
That's an easy one! Follow Paul's suggestion to (1) page through the text about 20 pages at a time and pick whatever jumps out, and (2) scan the index for words that have multiple references. I'd also suggest looking at the TOC and scanning headings for trigger words.
Relax, because there's no right answer here. Trigger words will help you latch onto ideas as you work through the text. Whatever you decide on personally is fine.
I'd also recommend that you follow Paul's suggestion to start with nonfiction materials because you'll find the structure (TOC, index, headings, subheadings, bolds, and so on) that will make this task easier.
--George
