First thought: If you are not already a member of your local library system, join immediately and learn how to access their Kindle books. Library systems are usually pretty slow to catch on that you've moved. Most library systems also have an extensive inventory of online MP3 audiobooks, if you roll like that (as does Audible.com, if you're willing to pay for audiobooks.) It's a cheap way to load up your Kindle.
Also, try thinking outside the Book Box by loading other types of horsey content onto your Kindle, like...
Learn how to deliver any text content from the web with Instapaper to Kindle. Think horsey blogs, horsey news articles from Horse.com/Chronicle.com/Savvy Equestrian/Sporthorse Nation/etc., I've even read long Kool-Aidy COTH threads on my Kindle. This article will show you how painless it is. It'll take you about 10 minutes to set it up, then forevermore it takes mere seconds. It's seriously slick, and totally free. http://www.hanselman.com/blog/Instap...gsAndMore.aspx
And to get your mojo thinking about which sites to pull articles from, try this list for starters. You might even start a COTH thread about top horse blogs for learners, or generate a list of trainers (Jane Savoie, Jimmy Wofford, etc.) who have blogs that you'd like to peruse for download. http://www.8womendream.com/4826/my-t...an-blogs-list/
and while there are lots of "search for free ebook on Amazon" crawlers, but I'm partial to Freebook Sifter because it cuts the BS/visual appeal and goes right to "here's the books, here's the rating, and click on this for a link to the book's page at Amazon.com": http://www.freebooksifter.com/?h=horse
Finally, I know you said you aren't comfortable reading in Spanish...but as someone who recently studied for a reading-in-Spanish translation exam 10 years after my last Spanish class in college, here's some thoughts on that.
First of all, you can change your Kindle's built-in dictionary to Spanish so that you can hover over any Spanish word and instantly get the translation.
If you'd like to look it over, I can loan it to you for 14 days through the user-to-user lending program. Just PM me whichever email address you'd like me to send the invite to.
To accompany the dictionary, there's plenty of "first Spanish book for readers" thingies on Amazon's Kindle search. I have the free First Spanish Reader one, and while the stories are not exactly riveting, I found them very readable even with my minimal Spanish. They repeat words over and over so that you learn them a little better. http://www.amazon.com/s?ie=UTF8&fiel...&index=blended
Best of luck! Costa Rica is beautiful. I've only been once to hike for a week, and I fell in love. That's from a girl whose family is from Hawaii, so it's not like I'd never seen a rainforest, but Costa Rica had its own special flavor. Don't we have some COTHer who keeps a horse, or at least used to, in Costa Rica?
One of my favorite books is Prince: A Year at East Hill Farm by Helen Husher. The description says (because I can't explain it any better): "A memoir about why we ride and why horses matter, framed by a year of joyful return to the complicated lessons of the horse barn. Instead of the how-to, the topic is why to, and how horses bring us clarity, thoughtfulness, and a new capacity for interspecies communication." It's really a great read.
I also have Learning to Ride, Hunt and Show by Gordon Wright and George Morris. It was $1.