'Gaseous Conductors' by James D. Cobine, McGraw Hill, 1941. It's one of those good OLD text books that really teaches you the theory and the practice. But good luck finding a copy. It took three years for me to find one and I had buy someone's entire library to get it! Despite the slightly misleading title, it goes into the characteristics of fluorescent lights, vacuum and partial vacuums, inert gasses, hydrogen gas (thyratrons), Mercury vapor, etc as well as the nature of the anode and cathode and space charge. Unlike radio tube books that deal with VT amplifiers and the like, this book covers every aspect of electron flow inside of tubes under many different circumstances. They do discuss radio type tubes but that is just one single use.