I'd also buy the entire AP in one go - like you, I want the whole thing, not just five of the six books. Having said that, the early instalments tend to sell out first so if I had to buy only part of an AP I'd focus on issues 1-4 first. I'd also prioritise the older APs I don't have over the newer (since there's an obvious correlation between selling out and time since release).
The pawn sets and interactive maps are less useful unless you're actually running the AP, so you might be better off waiting until you've read it (although there isn't a single AP I've regretted buying, there are some I know I'm never going to run, so stocking up on the accessories related to that AP might be a waste).
steve or anyone i was wondering what makes people give up on book one or two ? the maps are they dry markable? or can you not mark on them?
I'll let Steve comment on why someone might drop out of an AP after a couple of installments but I can tell you that the maps in the Poster Map Folio packs are NOT coated in the same manner as our Flip-Mats or Map Packs and as such, are not meant to be drawn on.
Gaming groups fall apart for various reasons. You're lucky to get thru one module before that happens.
steve or anyone i was wondering what makes people give up on book one or two ?
I don't really know as its outside my experience (I can't imagine buying the first few and then stopping, even if I didn't like it). However, over the years, I've heard a number of theories:
1. More people prefer low level adventures than prefer high level adventures.
2. Some DMs only buy adventures they're going to use. This means they might stop midway through when they realise an AP isn't for them.
3. Some groups break apart mid campaign. Personally, I'd read the entire AP before beginning to run it, but if you were "buying it as you go" and your group disintegrated, I guess you'd have no reason to keep going.
4. The early APs have the most useful back matter. (This isn't my opinion, but I've heard it said - gazetteers in particular were called out as "the most useful" articles and they're generally in the first few instalments).
5. There's hype around an upcoming AP which dies off midway through as hype for the next AP ramps up.
6. "Slack subscribers" may have no interest in an AP's theme and plan on cancelling their subscription but forget until after the first or second arrives. This is unlikely to be balanced by the opposite trend.
The final two reasons are just speculation on my part:
7. I've never heard it discussed, but I know Paizo print different quantities of each instalment (and different quantities of each AP). The decisions made their are probably complicated and could well have 'thresholds' determined by the printer. So, for example, if paizo predicted that the ideal number of product for each book was:20k-15k-10k-8k-5k-2k but their printer had price reductions per unit based on ordering at least 10k of a particular book, paizo might judge it worth the risk of higher storage costs to drive down the per unit cost of issues 4,5 and 6 by "overprinting" those instalments.
8. Similarly, there may be a policy of supporting all styles of play (or similar motivation), even if some are less profitable. So even though sales projections don't warrant such print runs, there might be other publisher goals which mandate "overprinting" some copies.