I use a few flexible criteria to discard reference material.
1) If I haven’t looked at within the last 6 months, there is a good chance it needs to go. I scan it and if I can’t come up with a solid reason to keep it, It is gone. I do have a few classic texts that I am keeping because they are the first texts I ever bought and just might be worth more than sentiment in few years. And If I haven’t looked at in over a year, there better be a good reason to keep. Otherwise, I would need a storage unit for my material. Not happening.
2) If the information is outdated or if I never applied it to what I am doing, it is long gone. I don’t keep any material that I haven’t found a use for within six months of having study it.
3) If I can easily acces the material online, I almost always toss the hard copy. And I try to see if I can get it digitally now instead of buying hard copies.
I always take my old material to the library first. If they don’t want it, I go to a used book store next. If they don’t want it, I try to recycle it. The trash is the last place I go to for almost everything these days.