There are many theories about Retrogrades planets, either takin each planet alone or the meaning of having much of them or none at all.
I encourage you to read as much as posible, investigate famous people charts and the chart of people your know and then make your own conclucions.
First of all is important to know that planets don´t move backwards it´s just an illusion for the ones who habit this planet, the earth. The fact is that this planets seem to move backwards when ever they are very close to the earth.
Schulman´s book explains them one by one and relates them to past lives. Allow me to introduce my point of view here, I do not believe in past lives (I respect people who think the other way) so I actually interpret past lives as attitudes we learn when we were little and that we need to fully understand and cimprehend and then, utilize it.
Erin sullivan has a great book also, about them.
If you do not apply retro planets to karmic astrology you may find then that each energy of each retro planet is taken and felt in a very subjective and personal way. I myself have 6 retro planets in my chart, the generational (Uranus, Neptune and Pluto) , the Social ones (Jupiter and Saturn) and Mars. My way to explain my point of view is very personal, actually I think that peolple with many retro planets need to understand everything applied to the planet at a very deep level, in order to internalize it and after that process, express it with conviction.
There is a difference maked with outer, social planets retrograde and personal planets such as Mars, Venus and Mercury. Sun and Moon never go retrograde. The Moon nodes are retrograde most of the time (that´s whay many astrologers relate it with karma) so the odd thing would be to have it direct (I did not investigate about this so I cannot add anything about this).
I have posted an artcile about Retrograde Mars, in Taurus and Libra (I have Mars Rx in Libra) it is interesting. Here´s the link:
http://www.astrologyweekly.com/forum/showthread.php?t=2364
I have found the finest writers and composers with mercury retrograde. One article from a national astrological magazine (it´s in spanish it would take a lot of time to translate it, but may be I could summarize and post it some day) explains points in the horoscope related to proffesional hability in astrology. Thay had investigated the chart of the most imprtant astrologers, in most of the cases (not all) Uranus was retrograde.
It is good to say this because many people say, specially those new to astrology that retro are punishment, or bad in a way. Wel, that´s not true and it may be a great gift. There is a book written by Joanna Wickenburg about interceptions an retrogrades, it is called "YOUR HIDDEN POWERS".
Here is an article written by Arlene Kramer that I found though a thread here in this forum:
http://www.arlenekramer.com/astrology10.asp
It includes information about tha aumont of rertograde planets in a natal chart. There is not a lot of information in this way, since retrograde have been investigated and theorized alone, and not treated in groupings. So there is a new topic for professional astrologers, or for you or maybe me to investigate...
Regards,
Mary