See my review of the first book in this trilogy, The Fellowship of the Ring, if you haven’t already.
One of two things is likely to happen to a “middle” book in a series: it will either be the best, unhindered by silly plot conventions (think Empire Strikes Back), or it will be a mere placeholder (think Harry Potter and the Half-Blood Prince). It’s difficult to say which of these happens to The Two Towers.
Technically, The Two Towers has some really important things happening, but some of that is obfuscated by Tolkien’s unconventional way of doing things. The book is split between the plot threads of Frodo/Sam and then all the rest of the characters in battle, but rather than weaving these two threads together, Tolkien literally splits the book between the two, first detailing the latter and the battle at Helm’s Deep, and then describing the former’s journey into Mordor.
And don’t be fooled by the grandiose battles of the movies (here I go again): Tolkien is entirely unconcerned with battle scenes, and the entire battle at Helm’s Deep merits only a chapter. Fans of the movie, however, will be happy to know that the snarky camaraderie between Legolas and Gimli is alive and well in the book version, and they do indeed keep tally of their kills during the battle.
Of the many themes that Tolkien coaxes out of this fantasy, one of the ones that really came to the fore was his general aversion to industry, and so Nature v. Industry was very much alive and well in the battle of Isengard—id est, the Hobbits and the Ents were nature, and Saruman represent the blind ambition of industry, turning Orthanc into the epicenter of a razed, oily landscape.
Of the three, this book may seem the most difficult to read, because there is not recognizable climax (again, the enormous battle at Helm’s Deep seems almost an afterthought to Tolkien), but rather two stories of indeterminable plot, back to back, a mere segue into The Return of the King.