I have heard/read that renting/purchasing rain boots for Uyuni salt tours might be a good idea. Where can one rent rain boots in Uyuni?
Renting just books is unlikely, although there's no reason why if that were a frequent request. Definitely, having rain boots to visit Uyuni is a good idea if during the rainy season. Although personally I went during that season, that year they were experiencing a drought, so it was quite dry! Buying daily items in Bolivia is very cheap, so it is possible to buy things things at a price that makes a boot renting business pretty tough.
The good news is that many tours actually provide boots. Perhaps not the cheapest but you certainly do not have to go on an expensive tour to get rain boots. You may not get exactly your size but common sizes are available. Here is an account of someone that did a similar tour recently for $25 USD and he noted that boots are included. I didn't go personally on that tour but it is similar.
Just an additional advice for your other extremity, in Uyuni I got a sunburn on my scalp for the first time in my life. The cold high-altitude air gives a false impression that the sun cannot burn but thinner air means rays hit harder. Hats on the other hand we not provided!