Occasionally on one of my book-hunting trips to the local thrift shops I will come across what was almost certainly the collection of a true student, collector and/or aficionado of a particular subject. Someone's entire collection of books on some topic will be on display, most likely having been donated after their owner's demise by a relative who didn't share their interest. There was a shop near where I used to work that I frequented, and once a large number of books about the Civil War, and mostly from the Southern perspective, appeared there, many of them fairly new and in quite nice condition.
I was poking through the books at a Goodwill store in a fairly upscale part of Scottsdale yesterday, and it soon became obvious that somebody in the area had had a serious if not obsessive interest in Charles Dickens. There were some copies of his novels, but mostly the hoard consisted of what I'd estimate to be between 150 and 200 books about Dickens: lots of biographies, collections of his letters and speeches, academic studies of his work (some published as early as the 1840s), etc. I found it to be both impressive and sad, as someone had obviously dedicated a lot of time in amassing all of that Dickens-related material, and it had all ended up on the shelves of a thrift store.
As for me, I only wanted three of the books - books attempting to finish or solve the mystery in Dickens' unfinished story, 'The Mystery of Edwin Drood', one of them printed in the 1890s, which has long been a sort of literary game for many authors.