What's a good resource/book on pH measurements? Most resources on pH and the electrochemistry behind it that I've found are IMHO too superficial. For example, they explain that an electrode with less than 80% slope should be discarded, but not why the decrease in slope happens, or why it's a problem if you can just calibrate it. Or they explain the potential generated at the Ag/AgCl eletrode and the glass membrane potential, but not the interaction between the two - does the H+ combine with the e- from the electrode? And what happens if the current is reversed? After reading for a few days I still feel like I don't understand any of it.

So far I've read a couple PDFs on nico2000.net (the 30 page Introduction to pH measurements), the 160 page book pH Measurements by C. Westcott (1978), a few manufacturer's guides to pH measurement theory and practice (by Radiohm, Emerson, Mettler) and the relevant chapters in the Analytical Chemistry books that my library had.

All help appreciated!
Looks like you understand pH measurement, but you are trying to go deeper into the details of potentiometry and open circuit potential measurements.

There I would recommend basic electrochemistry texts. Here is one simple book that is cheap, easy to read and very informational:

Electrode Potentials; Richard G. Compton and Giles H. W. Sanders