The midpoint of a planetary pair has no meaning unless occupied by another planet -- natal or transiting (including progressed and directed.)
Midpoint structures are generally of secondary importance. Aspects are, in most cases, stronger.
Reinhold Ebertin put out a book on generalized meanings of midpoints -- The Combination of Stellar Influences. It is the seminal work in the field. Michael Munkasey also published Midpoints, built on the work of Ebertin and his own experience. This book has the added bonus of showing how to use "keywords" in combination (by example, without explanation,) a very useful skill.
As with aspects, cookbooks can only give general guidance to interpretation. The midpoint structure must take into account sign and house, the lords of the three planets, the importance of the midpoint formation relative to the whole chart, etc. Each midpoint formation is very much "chart specific."
One way to approach the interpretation is to "read" the two outer planets individually first (giving meaning to "Mars", rather than just thinking "Mars"). It is meaningless to say, "I have Pluto at the midpoint of Mars and Moon" without understanding what Mars and Moon -- in the particular chart under study -- mean of themselves. Reading midpoints is complex.
I suspect you are studying midpoints prematurely. It is very important, before proceeding into more advanced areas of study, to first master the fundamental building blocks of astrology: Planets, signs, houses, aspects. These are the 4 basic tools of astrology and are far more complicated than most folks give them credit for (so they study them superficially and let it go at that.) For example, do you fully understand the Qualities (Modes) and Elements, of which each sign is a unique combination? Qualities and Elements are among the most fundamental and important symbol-systems in astrology, and study of them will yield far greater benefit (understanding) than study of midpoints, especially when still limited by lack of experience.
The Qualities and Elements (as one example) in astrology portray fundamental and universal characteristics of the person under study. The midpoints give superficial and specific information. Fundamental characteristics always predominate, and determine the expression of those horoscopic factors farther down in the hierarchy of power. They create "the context" in which the less fundamental factors will operate and to which they are subject.
Midpoints can be useful in interpretation even without giving them a specific interpretation. They can be used simply as pointers. For example, in my own chart I have a wide-spread and rather irregular grouping of seven planets. At one edge of this group is Mars, at the other Moon. The two planets are not in aspect. But at their midpoint stands Pluto. Now it is "as if" Moon and Mars are in aspect, with Pluto the mediating planet -- the "third party" who brings Moon and Mars together in common cause, whether for good or for evil. The midpoint formation brings the chart to a center -- it "points" to the central issue in the chart and provides the structure or framework on which the interpretation can be based. Because of its centralizing function in the chart, this midpoint structure is far more significant than other such structures in the chart. One of the problems in using midpoints is assigning each of them their proper place in the chart -- their relative importance or significance related to the whole. Not all midpoints are equal.
