So, as noted already, this time round my sense of Two Towers I was of a book artfully, and I think eloquently, passing from the tower of Death to the tower of Life (Gandalf, dead, revivifying; Theoden, a living corpse, returned to youth; Trees, rooted and insentient, transformed by Tolkien's imagination into roving, powerful Ents). Two Towers II, in complementary fashion, seems to me now to trace the opposite trajectory; from Life to Death, or some ghastly state in between which is not yet dead but not quite life. [It does not seem to me either irrelevant or random that the main characters of TT1 move, broadly, east to west; where Frodo, Sam and Gollum move, broadly, the opposite way, from west to east]. One way of summing up this book would be to invoke Coleridge's famous but, I think, poorly understood phrase: Nightmare Life-in-Death. That's what the book delineates.
Gollum is a major figure in this section partly because he embodies this Coleridgean fate: a creature who has lived far beyond his natural span of life and is more profoundly damaged and miserable as a result than is easily described. But the theme of the book hits home most powerfully in Chapter II, 'the Passage of the Marshes'. This is introduced by a clever little glance back to the Hobbit's riddles:
Alive without breathThe original answer to this ('fish') is joined, in this chapter, by a second, much more eerie possibility: for these words perfectly describe the warriors ('they lie in all the pools,' says Frodo, dreamily: 'pale faces, deep deep under the dark water ... grim faces and evil, and noble faces and sad/ Many faces proud and fair, and weeds in their silver hair. But all foul, all rotting, all dead' 653). These cannot be actual corpses, as Sam points out ('that is an age and more ago ... the Dead can't be really there'); but whatever they are they turn the very landscape into a place in which some grisly remnant of life clings to death.
as cold as death
never thirsting, ever drinking;
clad in mail, never clinking. 
As with the warriors alive-without-breath beneath the waters of the Dead Marshes (and as with Gollum himself), so with Frodo ... stung by Shelob at the end of the book, and so mortified that Sam initially believes him a corpse; yet a poisoned body in which life still clings. Shelob, also ancient, also more Death than Life (the 'stench of death' is about her, we're told;  and 'little she knew of or cared for towers, or rings, or anything devised by mind or hand, who only desired death for all others, mind and body' ). The Nightmare of Life-in-Death is very different, indeed profoundly so, from Death-in-Life. The latter is natural; a ripeness; the grain of existence. The former is a kind of violation. This book understands that.
This is why, I'd say, this is right place (in Tolkien's pattern) for Faramir's account of the Numenoreans. This quasi-Atlantean civilisation is JRRT's revisioning of Ancient Egypt, and an object lesson in pride punished. But the crucial details are the way the Numenoreans tried to cheat death, and created a population of ghastly mummy-like individuals as a result:
Death was ever present, because the Numenoreans still, as they has in their old kingdom, and so lost it, hungered afer endless life unchanging. Kings made tombs more splendid than houses of the living and counted old names in the rolls of their descent dearer than the names of sons. Childless lords sat in aged halls musing on heraldry; in secret chambers withered men compounded strong elixirs ...