I buy a fair amount of reference books I like to have with me. Several publishers offer .mobi file format, but you have to take an indirect route to get the book into your Kindle library.
A really simple way to do this on a device is to use Dropbox, which you probably already have. Upload the file, Let’s assume from a PC, then on your device, navigate to your Upload folder in Dropbox, or wherever you uploaded to, and then select ‘Export’. From here your device will list the possible destinations, and Kindle will be a choice if you have the Kindle reader app installed on your device.
In Android devices, the Kindle sideloader is already there with the Kindle reader and Dropbox has provided a direct path to export your mobi, PDF, etc files to Kindle. That’s right, you also get to choose the device you want the exported book ‘pushed’ to, so after a few minutes in most cases, the exported books will appear on your Kindle app carousel.