When we first started in this business our first book reading book light included a USB Male to Barrel Male cable, many of our customers complained that this was a rare and hard to find cable.
Although our cable had the advantage of being twice as long as those of the competition, they were right, this cable is inconvenient, for this reason we decided to evolve towards a Micro-USB connection.
Most companies that offer reading book lights are still using the barrel connection, if your cable was lost, it was damaged or simply want to purchase a longer one, here you can find the replacement.
Specifications: USB Male type A to Barrel Male, 78 inches = 2 meters, 3.5mm OD x 1.3mm ID x 11mm L (Plug measurements: 3.5mm Outer Diameter x 1.3mm Internal Diameter x 11mm Long)
Some of the brands whose reading lamps use barrel type cable are (Which brand is better?):