Purring would seem to be, in her case, an automatic safety valve device for dealing with happiness overflow.
I’ve already written about the treasures you can find at archive.org (books section), but holla, recently stumbled upon The Great Books Of The Western World. And yeah, they are available for free at archive.org:
Sniff, be still, my heart — yeah, having this access as a child would have been … well, yeah, no, probably not very beneficial:
«Possibly nothing at all; the overflow of my brain would probably, in a state of freedom, have evaporated in a thousand follies; misfortune is needed to bring to light the treasures of the human intellect. Compression is needed to explode gunpowder. Captivity has brought my mental faculties to a focus; and you are well aware that from the collision of clouds electricity is produced — from electricity, lightning, from lightning, illumination.»
The Abbe, when asked what he would have achieved if he had not been confined to a cell for decades, in «The Count of Monte Cristo» by Alexandre Dumas
But still, can’t help but purr (also considering the Harvard Classics are also available for free). BTW, if you have a Batch Link Downloader and expand the PDF section under download options, you can easily download all PDF files (use «.pdf» with «Add by filename pattern»).
Anyway, happy reading.