There are twelve volumes in Princeton University press's Physics in a Nutshell. For some reason I have eleven of them, not including Gerald Mahan's Quantum Mechanics in a Nutshell. I have no idea why I have so many, except that they look nice lined up on my bookshelf with their matching covers - except for the one I have in e-book version and my first edition of Zee's Quantum Field Theory in a Nutshell, which is different.
Mostly they are pretty good books, but they aren't Landau and Lifshitz or anything. On the other hand, they've got to be better than the endless Walter Greiner series I have tucked away in boxes. Of course, so is much of L&L.
So many books, so little shelf space.
Not only that, but I'm at an age when I need to be getting rid of crap rather than accumulating it.
From time to time I start collecting a few volumes to take down to the used book store. Too, too often though, I keep finding reasons why I need to hang on to this one or that. That out of date molecular biology book? Well, I could get rid of it if only I had the newest edition. Oops.
At some level, I suppose, we cling to things because letting them go means facing our mortality.