Try this Sonic Frontier site:
Look here: http://www.svetlana.com/graphics/tworks_fig3.jpg If you hold the cursor at about 5'oclock on the tube an enlarge button will appear.
The article above sited by Equinox is a good one too although it talks about the entire circuit more than the tube(which is good).
The getter and the "getter flash" (the "silver substance")that you mention are talked about in the first article below. The last article by John Harper has a little more math in it but you can read past it if you want and still learn quite a lot. It is nearly impossible to talk about how a tube works without numbers and measurements. You can click on the image of the tube twice* at the start of the first article at "Inside a tube" (almost at the start of the article) and it enlarges to give a pretty good view of things in a pentode.
1) How A Vacuum Tube Works, E. Barbour: http://www.svetlana.com/docs/tubeworks.html
2) Vacuum Tube Valley FAXs, E. Barbour: http://www.vacuumtube.com/FAQ1.htm
John Harper, Tubes 201, How Vacuum Tubes Really Work: a very nice modern and more detailed overview of what goes on in a tube. http://www.john-a-harper.com/tubes201/
Here is one about the sound of tubes: The Cool Sound of Tubes, E. Barbour: http://www.spectrum.ieee.org/select/0898/tube.html
Beyond a few cliches the sound of a certain part of a tube is very difficult to talk about and, as often as not, a waste of time attempting. Different types (845, 300b etc) of tubes have some general qualities but it is also the tube and the tube circuit and the operating points choosen, etc... all together.
Forgot this one on Getters.
* Werner Espe and Max Knoll, Getter Materials (from Electronics Magazine 1950): http://www.thevalvepage.com/valvetek/getter/getter.htm
Can't change the handle at this point. Fits like an old T shirt.