The second volume of Tolkien's epic trilogy never even wavers. If anything, it seems steadier and more controlled than "Fellowship of the Ring," as several characters become more central and the plot focus widens to envelop all of Middle Earth. It suffers from a bit of sequelitis in places, but the overall book is just as enthralling as the first.
Aragorn finds that Merry and Pippin have been abducted rather than killed -- for what reason, no one knows. Frodo and Sam have left on their own. So Aragorn, Legolas and Gimli race to find the orcs and retrieve the hobbits, but are stopped by the fierce Riders of Rohan, and then by an old and dear friend: Gandalf, who has been resurrected in the new form of a White wizard. Elsewhere, Merry and Pippin must use all of their wits to escape the orcs, and then find a strange band of allies that no one could have hoped for.
Meanwhile, Frodo and Sam head into Mordor -- with an eerily familiar figure, Gollum, following them. Frodo subjugates Gollum, forcing him to swear on "the precious" that he won't harm him. In return, Gollum promises to guide the two hobbits through Mordor, straight to Mount Doom. But the Ring is weighing more heavily than ever on Frodo, and is starting to reassert its old sway on Gollum...
One of the most noticeable changes in this book is the shift of focus. "Fellowship" was Frodo-centric, since the narration revolved around him, as did all the events and thoughts. But with the breaking of the Fellowship, the narration falls into three categories: Frodo and Sam; Merry and Pippin; Aragorn, Legolas and Gimli. This triple style allows individuals to shine more brightly, when they are called on to do more than hike with Frodo.
Tolkien also presented a wider view of Middle-Earth in general. While the slow slog through Mordor doesn't really tell or show readers much -- aside from what a hellhole Sauron is the middle of -- it's shocking to see the the effects of the orcs, Saruman and Sauron on places such as Gondor and Rohan.
But where Tolkien really outdid himself is Gollum. Gollum returns, in a substantially different state. Oh, he's still addled and addicted to the Ring, but he displays a dual love/loathing for the Ring, a weird affection for Frodo (who, from his point of view, is probably the only person who has been kind to him), and displays a Ring-induced multiple-personality syndrome. Very rarely can bad guys elicit the sort of loathing and pity from the reader that Gollum does.
One noticeable aspect of this book is friendship. When the Fellowship sets out from Rivendell, virtually everyone is a stranger, with the exception of the hobbits. However, in this book we get our view of how much Sam loves Frodo and wants to help him. Sam is fully aware of how much Frodo needs emotional support, and he's quite willing to be a pillar of strength for his friend. We see Gimli and Legolas's affection for Merry and Pippin; and Legolas's willingness to kill Eomer if Eomer hurts Gimli shows how far this Elf and Dwarf have come.
This book is substantially darker than "Fellowship." Frodo is starting to stumble under the weight of the Ring, and other characters die or are seriously hurt. The scene where Pippin's mind is trapped by Sauron is a very disturbing one, as is a violent and saddening scene late in the book. But there is also some wry humor, like Legolas's snippy comments about pipeweed as Gimli and the hobbits smoke up a storm, and Sam's debate with Gollum about whether they should cook the rabbits.
Tolkien's second "Lord of the Rings" novel is a thrilling fantasy adventure, exploring more of his invented world than "Fellowship of the Ring" did. "The Two Towers" starts heading into darker territory, and will leave readers panting for more.