The “form” of the soul that is suitable for its immortality

When the soul attaineth the Presence of God, it will assume the form that best befitteth its immortality and is worthy of its celestial habitation. Such an existence is a contingent and not an absolute existence, inasmuch as the former is preceded by a cause, whilst the latter is independent thereof. Absolute existence is strictly confined to God, exalted be His glory. 
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah)

When “the souls that are not good die, they go entirely away from this earth and so cannot influence anyone.”

Question: Are there "earthbound" souls who try to have, and do have, an influence over people, sometimes taking entire possession of their wills?

'Abdu'l-Baha answered, "There are no earthbound souls. When the souls that are not good die, they go entirely away from this earth and so cannot influence anyone. They are spiritually dead. Their thoughts can have influence only while they are alive on the earth. Caiaphas had great influence during his life, but as soon as he died, his influence ceased. It was of this kind that Christ said, 'Let the dead bury their dead.' But the good souls are given eternal life, and sometimes God permits their thoughts to reach the earth to help the people." 
(From a table talk by ‘Abdu’l-Baha, ‘Daily Lessons Received at Akka January 1908”, by Helen Goodall and Ella Goodall Cooper)

The “soul is a sign of God, a heavenly gem …whose mystery no mind, however acute, can ever hope to unravel.”

Thou hast asked Me concerning the nature of the soul. Know, verily, that the soul is a sign of God, a heavenly gem whose reality the most learned of men hath failed to grasp, and whose mystery no mind, however acute, can ever hope to unravel. It is the first among all created things to declare the excellence of its Creator, the first to recognize His glory, to cleave to His truth, and to bow down in adoration before Him. If it be faithful to God, it will reflect His light, and will, eventually, return unto Him. If it fails, however, in its allegiance to its Creator, it will become a victim to self and passion, and will, in the end, sink in their depths. 
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah)

The effect of gossip and backbiting on one’s soul

"... Backbiting quencheth the light of the heart, and extinguisheth the life of the soul." 
(Bahá'u'lláh, 'The Kitáb-i-Íqán')  

“The human soul is an essence spiritual in entity and material or physical in function.”

The human soul is an essence spiritual in entity and material or physical in function. It is defined as essence because it is independent in itself, while the body is accidental and dependent upon the soul. 
(‘Abdu’l-Baha, from a talk, quoted by Lua Getsinger in her talk in San Francisco on December 17, 1911)(To see the transcript of the entire talk please visit Baha’i Talks, Messages and Articles)

The “human soul” is that “all-unifying agency” that connects “the various organs and members, the parts and elements, that constitute the body of man” and “causeth them to function in perfect harmony and with absolute regularity, thus making the continuation of life possible.”

By nature is meant those inherent properties and necessary relations derived from the realities of things. And these realities of things, though in the utmost diversity, are yet intimately connected one with the other. For these diverse realities an all-unifying agency is needed that shall link them all one to the other. For instance, the various organs and members, the parts and elements, that constitute the body of man, though at variance, are yet all connected one with the other by that all-unifying agency known as the human soul, that causeth them to function in perfect harmony and with absolute regularity, thus making the continuation of life possible. The human body, however, is utterly unconscious of that all-unifying agency, and yet acteth with regularity and dischargeth its functions according to its will. 
(‘Abdu’l-Baha, ‘Tablet to August Forel)