The two most explicit references to Heavenly Mother are found is The Family: A Proclamation to the World and the hymn Oh My Father.
All human beings—male and female—are created in the image of God. Each is a beloved spirit son or daughter of heavenly parents,
In the heav’ns are parents single?
No, the thought makes reason stare!
Truth is reason; truth eternal
Tells me I’ve a mother there.
When I leave this frail existence,
When I lay this mortal by,
Father, Mother, may I meet you
In your royal courts on high?
Then, at length, when I’ve completed
All you sent me forth to do,
With your mutual approbation
Let me come and dwell with you.
Beyond that I haven't seen or read much on the topic of Heavenly Mother. She has been a topic of conversation frequently but all of it must remain speculation without more authoritative information.
I enjoy the speculation so I posted a response to this blog entry which speculates on why Latter-day Saints are not "allowed" to pray to Heavenly Mother. The comments are thought provoking and interesting.
Here is my response:
The speculation that my husband and I favor is that yes, each of us has a Heavenly Mother, but we all don’t have the same one. If polygamy is an eternal law, and one is trying to maximize reproduction of even spirit children, it stands to reason that Heavenly Father has multiple wives to whom he is sealed.
Also, on the ability to connect with our specific Heavenly Mother whether she is one of many or just one, I found comfort in a quote from Joseph Smith (of course, I can’t find the specific quote at this time) where he described it being okay to talk to beings who are not on this earth (living or dead).
After the death of a number of people close to me, I found myself turning in my prayers from a prayer to Heavenly Father, to a conversation with a loved one. I never knew if it was “right” or not, but its what I felt impressed to do. I got confirmation of that after reading the quote I’m referencing. It was okay to talk to the dead, knowing that they can listen and know what our hearts and spirits want them to know. If it can work for deceased loved ones and friends, why wouldn’t it work to talk to our Mother in Heaven?
Its not a prayer because prayer requires worship. When I talk to my father who is deceased, I’m not worshiping him but talking to him as if he were still here. My worship still squarely focuses on the God who I’m commanded and covenanted to worship.
The question that still remains for me is the appropriateness of worshipping a being other than my Father in Heaven (see my post on his perfect mothering attributes here). I will also try to unearth that quote. I think it might be recorded in my journal but journals don't have nifty search features (hence why I switched to a blog).
I had similar thoughts about my connection with Jesus. I have such a strong belief and connection to God the Father because I talk to him all the time. I feel like I know him through my prayers and the answers from the Spirit I have gotten in response to those prayers, and from priesthood blessings. Jesus, on the other hand, I do not pray to but he is my brother and my Savior. He knows me in such a perfect way because of the atonement. He knows all of my suffering and sorrow. Heavenly Father knows me perfectly but Jesus knows me too. I want to feel that I know him like I know Heavenly Father as well and by extension how I would also like to know my Mother in Heaven.