OneRedRook
Explorer
So, with all that said, what things should I concentrate on putting in such a reference booklet and what should I avoid?
So, uh, first-time-long time. Hi, Enworld!
There's been some really good advice in this thread, hopefully this post adds a bit more.
Quickleaf had an interesting point I want to add to. Players have a formalised way of hooking into background info by way of Knowledge checks and the like. It might be worthwhile creating reference broken down by relevant skill, so when a player asks "Can I make a knowledge check to see if...", you can say "sure, but before you roll here's what you already know".
I'd suggest avoiding a large Wall of Text for these, but rather break each check down into small sections by topic within each "knowledge". This allows the player to quickly get to the point without holding up play, and it lets them to catch up on this information as they become interested in it (and thus more likely to remember it). Also, players can get an idea of what else is there as they skim the page, and follow up on that if / when it starts to become relevant.
Another thing to consider is that when players are just starting a campaign, amongst all the things they're doing there are three which I think are worth pointing out. Firstly, they're trying to find the adventure. Secondly, they're feeling out how their character ideas work with that adventure and setting. And thirdly, they're testing the various interactions between their PC and other PCs and NPCs.
Anything information that's not directly relevant to the current situation is probably competing against those three processes, and as a GM you don't want any of those things to lose out. So before the first session, I'd suggest give them only the essentials for their character concept (eg, racial naming conventions, basics on their chosen faith, etc), and waiting a couple of sessions or so before starting with the "here's some general stuff you know" handouts. Give them a bit of time to get the hang of the campaign, and to get excited about it. If you went with the skill reference idea above, then this might be a good point to give out those bits of information which don't naturally lend themselves to being catagorised in a skill. But again, prefer being relevant and concise over breadth of knowledge.
Lastly, just to re-iterate a previous poster, props are cool.
Even if you don't think these ideas will help, I hope they help clarify what you do think will work.
Hroc
(Who knew Wall of Text was so high level?)