If you travel a lot, like most of the wildlife conservationists I support, then you will likely want a Kindle or other eBook reader. Amazon makes its easy to manage books purchased on the Kindle or at amazon.com, but not so much for books you aquire outside their ecosystem (like a PDF for example).
That's where Calibre comes in. It's great at converting between formats, and for pushing eBooks from your computer onto your Kindle. It's free (yay!) and looks to be a passion project by programmer Kovid Goyal. If you like it and find it useful, I recommend donating to him to thank him and encourage him to continue supporting it going forward.