I would love to own all army books, but unfortunately I must answer B here: I just don't have enough money to buy army books of armies I won't play. I own 3 army books: High Elves (I almost collected them, and I still love them), Bretonnians (my main army) and Wood Elves (I once thought about having WE allied contingents).
My favorite army book is Warhammer Armies: High Elves. If I had to buy another, it would probably be the Hordes of Chaos army book.
I have the Hordes of chaos and beastmen book, both the old and new dwarf book, the old Orcs and Goblin army book, the Tombking book and the wood elf book.
I have pretty much all of the army books. A lot of the 6th edition ones and all of the 7th edition. A few 5th edition aswell. I dont think they are that expensive to buy, since GW release around 2 armies or so a year. I have the new empire book, but I have not had the time to go through it very thoroughly yet. They are like any other books that seems intresting to me, IŽll buy them. Also, me and my brother have lots and lots of armies so naturally we have almost all the books aswell. I enjoy reading them B) .
I used to own all of the army books when I was playing 3 tournaments a year. Now I mostly play gothic and bloodbowl I only own the ones that I need.
I had all the 6th edition ones, bought them when they came out. Now thats seventh edition has come my collection has been lagging a bit.