I got all the WAPJ volumes after they were published, got them from various airshows over a period of time, all brand new from £3 to £6 each..
Same with the Wings of Fame series really apart from the hardback one( Which was also my first one ) which was a gift from my sister.....she found it in a charity shop for £1 , the rest were from shows around £4 each.....
I got really lucky with the IAPR volumes, found them on ebay (15 volumes ) for £20 delivered, still need to get the rest but I've not seen any in my price range....yet.
I've always been a bookworm & spent silly amounts of money on whatever I've been intrested in at the time, be it American cars, trucks, military vehicles......the list goes on & on only to sell them all ridiculously cheap when my intrests changed.........I wont be doing that again..
I only have 50-100 pdf type books, mainly flight manuals & stuff like that, cant beat real paper in your hands..
You can NEVER have too many books.