Price increase in rare valuable content books is to be expected; it is an economic law of supply and demand, the supply is low and the demand is high the price will increase.
For over fifty years I have been a diving bibliophile and have created diving library of over 2000 volumes plus 13 bound periodicals which is considered by serious diving bibliophiles as the largest and most complete collection devoted to recreational diving in the US and probably the world.
Almost 20 years ago I organized and presented the very first US diving bibliophile meeting at my CenCal home which was attended by serious diving bibliophiles from the western US and via telephone to others within the US who couldn't attend in person.
Many predictions came from that first gathering: dive books would soon fly off the used book store shelves, the price of bibliophilic quality collector books (intact dust Jacket, no rips, tears or markings and minimal foxing) would become increasingly rare and increase exponentially in value, Books that could be purchased at that time for a few dollars, if they can be located now the seller will demand antique collector prices or more appropriately they can not be found or purchased at any price.
It is not difficult to comprehend the sudden and excessive price increase in Sheck Exley's books = supply and demand. When published there was a great supply of his works and minimal demand, now that the world of recreational diving has increased like two rabbits in a 3 foot cage there is an excessive demand for his words of wisdom, therefore price the increase.
But how many of you have read or heard of "Caves and Cave Diving" ?? Authored by Guy De Lavaru, published in 1958 in the US by Crown books. It is the fountain head of cave diving books and it is currently available at reasonable prices.
SDM
