I'm also a Kindle owner, and found that it's a fantastic machine to read books. Real e-ink is 100000 times better than ANY LCD device on the market concerning reading comfort. It really feels like a real book.
But perhaps not the best for most of the wargame rulesets you can find out there, since they are designed for A4 (or us letter) format, dual or triple column, diagrams, textured backgrounds, etc...Not very confortable on the usual 6" or 7" screen of a e-ink e-reader.
The other problem with rulesets is also that you often need to go from a page to another one, go to tables, index, summary, etc...Not very easy on any digital device (tablet or e-reader).