Pathfinder NPC Codex (& other books) in a 3.5 game?

[Zorgoth]

I've read a lot of good reviews of Pathfinder's NPC Codex and I was wondering if their would be any difficulty integrating it into a vanilla 3.5 game. I'm considering purchasing a used copy as unlike most Pathfinder books it is quite cheap.

I'm also considering whether I want the core rulebook and gamemastery guide. I already own 14 3.5 books including the three cores (as well as the core 5e books).

Note that I can't use the d20pfsrd because I'm a religious Jew and run my campaigns on Shabbat, which means no electronics.

The Core Rulebook is obviously handy if I want to replace weak classes with their Pathfinder versions.

I'd be interested in the Gamemastery Guide if it contains a lot of useful advice for designing campaigns that I won't get from the DMGs of 3.5 and 5e.

I don't know exactly how much of the goodness is contained in Core and how much is in the other books, however. I can't afford to buy an entire library when I'm only using Pathfinder material as addons to a 3.5 campaign (I might run full Pathfinder campaigns at some point in the future, but not in the near future).

[Psyren]

The NPC Codex is just a collection of statblocks and artwork; there's no rules in it. What exactly did you want it for?

I know you said no electronics while you play, but given that you're presumably online now, I would suggest just printing off the statblocks you want to use and putting that money towards a different book like Ultimate Campaign, which also has GM'ing advice, or Ultimate Equipment for treasure tables, since that sort of GM assistance seems to be one of the main things you want.

The only practical way to tell what "goodness" is not in core is for you to browse the PRD (http://paizo.com/pathfinderRPG/prd/) and see what the other books contain (archetypes, spells, feats, items etc) that interests you. I recommend the PRD in this case because the info is sorted by book, so if you find a book that has a high concentration of stuff you like, put that one on your buy list and then you can reference it during your games on Shabbat.

[Zorgoth]

My idea was that it would be useful to have this book for emergency situations where I had too much real work to do to focus on preparing all the content I'd want for my campaign. The book is quite cheap compared to other Pathfinder material, so I thought it might be worth it for that purpose. The reason for having the book as opposed to using the web is that I can look for statblocks after Shabbat starts (and thus when I'm not doing work).

[Psyren]

If it's just a random NPC you might need on short notice, then yeah, having the whole book at hand sounds like it would be a good investment.

For your other question, see my last paragraph.