Oh man, I can't like this post enough. The Rum Diary is my favourite book and one of three books that I repeatedly read every year. I have a first edition of both the UK and US versions, although it was only released in 1998 so not quite as rare as his others. I also have a beat-up reader copy that I read each year and scribble a tally on the front page, the current count is 9 I believe.
One of the reasons that I like it is because it was written at the start of his career, just before Hells Angels and 10-ish years prior to Fear And Loathing. You can really see from the writing how he begins to see the evils in the world and the worst in people, which kind of set the foundations for his whole career. Whilst the Gonzo style is less apparent with the Rum Diary, I actually imagine a lot of it is based on Thompson's early life. I'm sure you know, but the cover shows him sitting on a beach with a glass of drink in one hand and a pencil in the other, with a new hotel in the background. It's pretty easy to imagine it being Paul Kemp rather than Thompson.