You can either email books for free to your kindle using the
yourname@free.kindle.com adress amazon puts at your disposition, or you can download books with the kindle browser from the calibre server if you use the calibre freeware, which has a dedicated sub forum here :
https://www.mobileread.com/forums/forumdisplay.php?f=166
For those two mode you will not need your usb cable, I use both all the time and it works really well, and it is the simplest way for me. If you use the calibre server solution, just remember to keep calibre (and evidently then the PC) running.