The Pink Shirt Book. What would I have done without this book? If you were programming to the "IBM PC" at the time, this was a must have. I pulled it off the shelf and it flopped open the chapter on INT 21. This was the system services interrupt the keyboard, files, etc. I also found a dog-eared page for the INT 27 terminate and stay resident (TSR) details as well. TSRs were the services of the time. This was mostly assembler, then bumping up to C as soon as I could.
The Genie Book. I used this book in conjunction with the Norton Pink Shirt book. It came out after the Norton book, but had more details on writing TSRs, multitasking, and expanded memory. I pulled this book off the shelf and there is a page that was used so much that it was sticking out - having come off the binding. Now that is one well used page. When grabbing the image for this post, I found that this book was available on the Internet Archive (archive.org). E-versions of technical books is one reason I was comfortable getting rid of so many of my technical books (see future purge post).
The Microsoft C 5.0 Quick Reference Guide. This is an actual picture of my copy because I could not find a stock photo. Note the cup stains on the cover. It was a C reference, the page I used the most for C were the escape sequences for printf (e.g. \n, \r) and the language type ranges (e.g. difference between long and float). I still use this reference occasionally for the last table on ASCII codes - the decimal, hex, and character representation of 0 to 255. If you have seen The Martian, then you know.
The Intel 386 Microprocessor Hardware Reference Manual. Yikes, I am not sure why I am hanging onto this one, I have not used it since the time when I needed it. I have flipped through it a couple of time since and seen all the timing and logic diagrams and thought - we actually did this stuff. I primarily used this around interrupts (maskable vs. non-maskable) and for figuring out peripheral timings for external serial devices. We were doing some pretty hard-core stuff at the time. I recall a device we borrowed from one of the hardware manufacturers that sat in between the CPU and the socket. It allowed you to debug at the CPU level and single step through code. I remember we called it the skateboard because it resembled one but have not seen or heard of one since. Hardcore.