You have some good answers already, so I am going to focus on one section.
The life of a book printing.
(we are talking traditional, not print on demand.)
I am going to use the words publisher, printshop, distributor and retailer. The same business may be multiple of these.
First the publisher does a print run (by negotiating with a print shop).
This likely takes several months (printing books is quiet slow).
Even today with colour laser printers, setting up (reconfiguring) a print run takes time (and money). So when you start a print run you want to print all the books you are going to sell at once, because it is not as cost efficient to start it up again later - but on the other hand the contract with the print shop is for some fix amount of time (==copies), they have other things they want to print (for other publisher clients).
The publisher will warehouse some of these and ship them to the distributor.
So distributor (wholesalers kinda) who stick it in a warehouse.
Depending on the print run length the books may ship before printing stops.
Once the publisher's warehouse is empty and the print run is finished, the book is now out of print. Distributor can't get new books. Most books will be out of print even when they are still fairly easy to get from retailers. Most books are out of print all time.
From there retailers order from there preferred (perhaps only nearby) distributor.
This process takes a while, transit etc.
If it is selling well it might get its print run extended (by the publisher negotiating with the print shop).
More likely by the time they realise, the print run will be over.
So they may decide to do another print run (Go to top).
If the publisher does do a new print run, it will be many months before they hit the store.
Never bank on a reprint. The publishers, and retailers like to get your hopes up as it will keep you coming back (and because they love the product too, and project there desires for a reprint. If one is occurring, it will take so much time it is not worth waiting. If they say we will be doing a reprint in 2 months, then it will be more like 6 months before the book is in your hands.
Now from the other perspective, the retailer.
A customer walks into store and says: "I would like X, have you got it?"
After checking out back etc and finding that they do not, the retailer checks a number of places.
- If retailer is part of a chain, that has other nearby branches, they can check if there sister stores have it in stock, and either redirect the customer or get it posted to this branch.
- They contact their distributor-- set if there are any left in the warehouse.
- The distributor, will check their other warehouses, (this is where things can happen, like they have a copy sitting in a warehouse at the other side of the country.)
4.) They check if they have any coming in, either though the publisher, or via a buy back (see below).
- the distributor will also contact a higher level distributor (probably the publisher) to see if they have any.
If all this fails (not unlikely) then the next step is a Out-of-Print/Rare book dealer -- possibly a service offered by your retailer (This may be the "Special Orders" desk. Dymocks in Australia has one, though I haven't tried it on RPG books)
What these people will do is find out if any other retailers have a copy of the book unsold. They will also contact international warehouses (some normal retailers might do that too.).
A Out-Of-Print/Rare Book dealer may be quiet expensive. The Special Orders department, will not be notably different from standard retail (but it is a difference in the effort gone to.)
If the book does not sell out at the retailer after a print run, buy-back may occur (This will be part of the sales contract, basically "we promise that if this doesn't sell, the retailer loose to much" so it lowers the risk).
The redistributer may take back the unsold books to sell to another retailer.
If things go really badly, the publisher may buy unsold boxes of books from the distributor.
There are Remaindered Books.
Many go to Discount Book sellers -- alot of which are also second hand book stores. So some "second-hand books" may be brand new.
Some get given away at conventions, or donated as prizes.
I've not seen much buy back or remaindered RPG books (but I am sure it happens).
Most unsold books tend to linger in the deeper darker corners of the retailers -- have a dig in some of the corners of your FLGS, I'm sure you'll find some GURPS 2nd Ed or similar.
A few final thoughts on the question as a whole:
- As I said: Never bank on a reprint. Its just not with the time.
- Be careful buying second hand on line. Sometimes a book goes though weird bubbles (and/or people trying to scam) and may cost orders of magnitude more than origianlly retailed. nWoD:Geist was abouut $60 new, when it went out of print people were trying to sell it for up to $1000, sure that was the extreme end, but it going for $100-200 was not unusual.
- Old content may become available as Print On Demand, or as a downloadable pdf.
I don't have any professional experience with this.
I would love to get comments from retailers, redistribute etc who know the process better, and I will update the answer.