I dunno - I have to agree with this statement. If I had valuable enough books to necessitate an entire safe to store them - I'd probably have them in a safety deposit box. I wouldn't keep anything mega-valuable stored inside my house.
But, back on topic - the question I have is why the preference of a large fire safe compared to a large gun safe?
According to this fire safe article , there were 389,000 home fires reported in 2008. According to the US Census, in 2010 there are 131,704,730 homes in the US - using those two figures (despite the variance of the two years data), that is 0.29% of homes succumbed to fire. That seems pretty long odds for fire.
A four-drawer legal-sized fire safe is $4,460. And it's not waterproof. I watched a building burn downtown where I work and saw fireman put water on it for 12 hours because of the heat. And it's not like the owner of the building was able to walk through and pull out his valuables. I'm going to guess that if your house burns to the ground, the biggest, largest, longest rated fire safe might not hold up to the heat (especially if it's in the basement where the structure would collapse, taking the slow-burning heat with it) and even if it did - I can't imagine that the residual heat wouldn't damage the books, nor the amount of water used to put out a fire would not make it's way into the safe.
If the desire for a safe is simply peace of mind - I would guess a big gun safe with a de-humidifier bolted to the cement from the inside might be the best financial choice? Slap on an insurance rider for your books and that might be enough coverage?
Or, if you have $4000-$8000 for a safe, maybe considering a lockbox type of room that is completely insulated and waterproofed might be the better choice. An aquaintance of mine had one formed into his house when he had the foundation done. It was a separate all-cement room that he raised 6 or 8" above the floor and put a commercial-grade steel exterior door on - he didn't collect comics, but he had lots of coins, gold, guns and things of that nature.