I have found a way to transfer books from my android phone and my android tablet to my Kobo Libra wirelessly but I am sure it will work with any other Kobo however you do need to downloaded the (free) app HFS (Http File Server by vortexinfinitum) from the Play Store to your android device and give the app access / permission.
On opening the HFS app it gives you an address (which is the same address each time for that device). Then, leaving the app open on your android device, open the Kobo web browser *** and type in the address from the HFS app as if it is a URL and click on GO. Your Kobo now has access to your android device. Just find the book you wish to transfer (mine can be found in my Downloads folder) and tap on it. Kobo will then ask if you wish to download the file (which you do so continue) and once downloaded voila the book is now on your Kobo. You can only transfer / download one book at a time but you do not need a computer to transfer the book from your android device to your Kobo.
I have actually saved the address from the HFS app as a Favourite in my Kobo web browser as it is the same address each time for that device. Each device uses a different address so I have in fact two Kobo web browser Favourites; one for my phone and one for my tablet!
*** On the Kobo Libra to open the web browser from the home page click on More / Beta Features / Web Browser / Start.
Happy reading, Hilary