Since I'm still working on Ranks of the Undead and also thinking forward to the ethereal monsters and psionics PDF, I thought I'd ruminate on how I handle spirits and souls.
In ancient Egyptian lore, humans aren't just minds living in bodies. There are actual multiple parts to the nonmaterial side of a person. I think everyone has something like seven souls, but I don't recall the number. But in particular, there are two, the ba and the ka. The ba is the personality and takes the form of a bird with a human head. The ka is the vital essence and is what goes away when the person dies.
I've seen some examples from other cultures that use a similar two-fold division, but re-assign some feature. The more emotional "animal soul" is the one that leaves the body in the form of a bird, butterfly, mouse, or other small creature, while the personality and essence of what the person was survives as a ghostly image or shade. The animal soul is the vital part, the spirit (Greek pneuma,) and the intelligent part is what we normally call the soul. AD&D seems to use this same distinction, or at least distinguishes between spirits and souls. Occultism and theosophy get a little more complicated and distinguish between the spirit or ethereal double, the astral body, the mental body, and sometimes the causal body. But we don't have to go there...
Anyways, I follow this pattern, but keep it simple: the spirit is the ethereal part. When a person dies, the spirit typically dissolves into ethereal particles and that's that. However, the spirit is connected to emotion and passion, so strong emotion can sometimes allow the spirit to continue its existence even after detaching from the body. It may maintain the appearance of the person at first, but the distinctive features tend to erode. It may retain some memory, usually about whatever strong emotion created it, but the spirit is at best semi-intelligent and really no longer the original person.
What you would call the person exists as an astral image. When the ethereal part or spirit separates from the body, the astral part or soul loses all connection to material existence. It can only be seen in dreams or by those with psychic abilities, and usually eventually gets caught in some dream-world. It can't return to a body without some kind of spirit. The Raise Dead spell repairs the original spirit, which usually lingers near the body, while the Resurrection spell rebuilds a new spirit to forge a new connection between body and soul.
I'll have more to say in a future post.