This has taken so long - I didn't really have a handle on this lady! And still don't but I shall try and give my impressions.
First off, when I saw the image Moongold had posted, I was baffled because she seems totally expressionless, and I couldn't understand her. When I received the deck a couple of weeks ago (love it!), I studied her in my hand: I thought I would see more. But she remained hermetic. Well - she is the High Priestess. I read a lot about Isis. Then I read the book that came with the deck and it wasn't quite what I had read in my mythology books, so we have have different sources, Clive Barrett and I. I was especially annoyed that he'd given Hathor, the Empress, some of Isis' legends!

(of course Isis and Hathor are the same goddess, or at least became so in time). But the lady on the card still remained silent.
I wrote nothing because I had nothing to write.
Then I did a 6-card reading for myself. I don't know why it was six cards - it was random, I stopped when the circle felt complete and assigned no "positions". In this reading, the High Priestess, Isis, came up next to Hathor, the Empress. And exactly below them - the Moon and the Sun.
And it clicked! Or at least, something did...
Isis is a sorceress, a powerful magician, a knower. She extracted Rê's real name from him with a magic ruse, which made her the most powerful goddess. But she ain't telling. She is also the sister, lover, wife of Osiris. When he was killed by their jealous brother Seth, she kept his body in a sarcophagus, but Seth found it and tore Osiris to pieces and scattered him along the Nile. So she searched the long river, brought together the pieces of her love and hid in the Delta reeds with him, and wept and sang him back to life. Here is some of her magic song, translated from the hieroglyphs (not by me!):
"I desire to see thee again,
I am thy sister Isis, whom thy heart loved,
Pursuing thy love when thou art away.
I flood the country with my tears, today.
Come back in peace, oh my Lord, so I may look upon thee again.
O sovereign, come back in peace.
I am a woman good to her brother,
Thy wife and thy sister.
Come back to me in haste,
For I desire thy face since the time I was deprived
of gazing upon it.
A heat is in me because of my love for thee.
Ah! Come that I may look at thee!
Darkness dwells here for me, in my eyes,
Even when Rê is in the sky
My heart burns to be separated from thee, o woe!
I lament thy love for me.
Come back! Be not solitary, be not far away!
...
Come alive, come alive!
Thy limbs once more belong to thee, o Wennefer!
Thy flesh once more belongs to thee,
o thou whose heart was weary.
Those in the Underworld welcome thee in joy.
Stand up! Stand up in peace!"
He stayed alive long enough to make love to her and make her pregnant with Horus.
The High Priestess has at her feet offerings of pomegranates and camels - essential items in the desert, guarantors of life and the continuance of life - almost magical gifts. A dove shows the spirit that inhabits her - her love for Osiris, her magical gifts. She wears the crown of Hathor, because she is also Hathor. Mother - not of a born child: she is the gestating mother, the lunar mother who is preparing creative gifts and children in the dark of the womb or the mind. She is the bringer of life, for it was Isis who with her tears of love brought the floods yearly to Egypt, which fertilised the soil along the Nile with rich silt.
Below, her, the card of the Moon reminds me of her lunar attribution - she stands at the gate of the land of the Dead, over which her beloved Osiris now presides. The Moon, like the floods that blessed Egypt before Nasser built the Asswan dam, is cyclical - an eternal beginning, middle and end...
Hathor/Isis, whom we see enchantingly in a Garden, is the mother of Horus, the born child, the child of light and sun, who eventually claimed the heritage of Osiris, his father, and became the overall sun god, the hawk-headed, god of war and life. Hathor is the Sun goddess, the warm daylight side of Isis. She has birthed the creative and real children of Isis, she is busy developing them freely under the sun. The Sun is not cyclical, but back then it was believed to be - it rose every morning and set every night - more or less at the same time year-round, in Egypt, give or take an hour. A much quicker cycle than floods...
The High Priestess can tell us to be patient. A child or an idea needs gestating, and all things - full Moon, floods - come in their time. She is telling us not to talk of our unborn children to all and sundry, but to nurture them in the darkness, in the moonlight. She teaches us magic, the magic name of the Divine, if we have ears to hear her silent voice...