I found the one on queueing (NOT "queuing" or "queing" or "quing" or "qing") theory outside a colleague's office at work. When lecturers retire, move or have a clear-out, they often leave books they don't want in the hallway for people to loot. I looted this one because in the past I'd looked at how to handle command input queues in MUD and could have found it useful then. I reasoned that I might find it useful in the future. I didn't, and now there's the Internet to tell me anyway.
I bought the one on business plans for real money. I'd just written a business plan and was told it could be improved. I purchased this book, applied its principles, and made my business plan even worse. It went through 14 rewrites before it was good enough. I kept the book imprisoned to protect the public, but it's so out of date now that it's no longer a danger.
The Advanced Engineering Mathematics book is the one all my fellow undergraduates had to buy if they were on the hardware stream of the BSc Computer Science. People would panic if they didn't have their Kreyszig with them, as it meant that the lectures would be incomprehensible. Hearsay evidence suggests that many of the lectures were incomprehensible regardless. I think I bought this copy second-hand from the university student bookshop. Its content is still solid, but we have computers to do Fourier transforms for us nowadays. I chose the Numerical Analysis stream instead of the hardware stream, so my essential textbook was Broyden's Basic Matrices, as recommended by our lecturer, Professor Broyden.
The HTML book was bought new. I got a lot of use out of this (you can see the page markers peeking out of the top), but most of the content is now deprecated. Putting it in the recycle bin has saved me 2½ inches of shelf space.
I do have some other books I could dispose of, but I'm keeping those to sacrifice the next time my wife comes along and orders a purge.