Ken (NOT Cookie!!) Sewell's book was based on a misreading of a mission by the Trieste III bathyscaphe* in 1972. The Trieste did a dive just 300 miles from Hawaii, coincidentally to 16,400 feet, the same depth as the K-129 wreck. The dive was an attempt to recover the film capsule from the first KH-9 satellite, which had a parachute malfunction. You can read about it here: Underwater Ice Station Zebra The K-129 was located at 40.06N by 179.57E.
As far as what was recovered, I have spoken to several people on the mission. The 34 foot bow section recovered did not have any code materials. I have a partial list of the materials (about 15 pages) and it is mostly listing items like electrical switches, valves, and other pieces of equipment. The interior was a mess, with most of the equipment stripped from its installation point and in a 3 foot deep pile on the deck. Human remains were mixed in to the debris. Materials from several compartments aft were found, indicating a massive pressure wave that swept forward, partially blowing off the bow. One bow compartment was collapsed down to a much smaller length and a few crew remains were found that were reasonable intact and allowed for identifications. At least one torpedo with nuclear warhead, badly mangled, were also recovered.
The failure of the CV arms was a result of being fractured when they were rammed through the seabed under the K-129 target to get under the wreck, which caused the failure of at least three arms and davits supporting the aft section. The seabed was much more compacted than calculated. More weight was offloaded from the Glomar Explorer Lift System to ram the arms and davits into the seabed.The davits had water jets to loosen soil at their tips, but this wasn't enough to make the soil penetration significantly easier. The highest value object recovered was probably was the recovered and mangled torpedo with the nuclear warhead.
* I know, the link below says Trieste II but it was really Trieste III. Trieste II was a modification of the original Trieste with a new float, shaped somewhat like a boat. Trieste III was an all new design, and the original Trieste II was disassembled and the all-new Trieste III given the Trieste II designation as a cover story. Trieste III was originally for "Winterwind" a recovery effort of Soviet missile parts and other "interesting" things on the ocean bottom. But other needs led to its "black" existence being revealed. You can read the whole Trieste history here: "Opening the Great Depths.
The Bathyscaph Trieste and Pioneers of Undersea Exploration" by Norman Polmar and Lee J. Mathers
