Restricted collections are always amazing to browse. In the Bay Area, for example, the Bancroft Library at UC Berkeley is a fantastic resource for browsing extremely old and rare books. I believe it's open to the public but you have to register[0], and you have to specifically request the items from circulation.
Want to see a Mark Twain manuscript, or an original Dürer print? You will be able to "check" these items out in a time-box, and browse them on-site. It's amazing how available these resources can be, and not many non-academic researchers assume you can see these items with adequate permission.
Having been to many rare book conventions, I've seen the librarians from the Bancroft Library purchasing items often upwards of tens or hundreds of thousands of dollars. The rare book world is very fascinating.
There was something of a game to identify a volume that was in the Doheney library's restricted list at USC. It took a bit of social engineering and the knowledge of a title in order to get the librarian to even let you know if the volume was or was not part of the collection.
That said, libraries that take their archival mission seriously (as all good libraries do) will always have to restrict access to volumes that are at risk of damage or theft for moral, political, or value reasons.
It was the University's main library so there were lots of different ones. For a while, one of the interesting ones was the congressional report on the nature of pornography (it was illustrated) but that eventually made it out into the main stacks, I remember a book on the McCarthy anti-american hearings which had some material on people in Hollywood that was "sensitive", some chemistry books that were pretty explicit about early explosives and their manufacture. That sort of thing.
As the article states, and I doubt this was different about USC, restricted books back in the 70's and 80's often seem tame by modern standards.
When I worked in a college library in Seattle, we had a single book that was restricted so that the library patron had to read it* while supervised by a library employee: a out-of-print, full-color field guide from the 1970s to Pacific Northwest mushrooms.
I'm guessing the reason for the restriction was the risk of people stealing it or razoring pages out - something that happens with some frequency with out of print art books.
It's even more of an issue with books which have images of obscure flora and fauna, at least with an art book if there's demand for a really good picture of a sculpture or painting you can pay someone to go take one, with pictures of rare mushrooms you'd have to go to huge lengths just to find them let alone get good pictures of intact specimens.
The Yale rare books library[0] is a beautiful, amazing building to walk around in. Most of the books are off-limits for fear of damaging them, I think. (This is what originally sprang to my mind when I read the title of this article.)
The British Library has a complete collection of UK newspapers going back to the 19th century and beyond. In the latter half of the 20th century, there were a few tabloid newspapers that routinely included photos of topless women [1] as part of their value proposition. When this began, such pictures were legal if the woman in the picture over the age of 16, and sometimes, these newspapers used pictures of women that young. In 2003, the threshold age changed to 18 [2].
Restricted stacks are one thing, but having thousands of items scattered throughout your open stacks suddenly reclassified as child pornography, according to particular details that probably aren't even in your index, must be a librarian's nightmare.
OTOH I doubt you'd have to look too hard to find a group of teenage boys willing to go through every single page 3 and identify those that were described as being 16 or 17. You probably wouldn't even have to pay them :-)
Want to see a Mark Twain manuscript, or an original Dürer print? You will be able to "check" these items out in a time-box, and browse them on-site. It's amazing how available these resources can be, and not many non-academic researchers assume you can see these items with adequate permission.
Having been to many rare book conventions, I've seen the librarians from the Bancroft Library purchasing items often upwards of tens or hundreds of thousands of dollars. The rare book world is very fascinating.
[0] http://www.lib.berkeley.edu/libraries/bancroft-library/plan-...