I have just finished watching Season 2 of The Shannara Chronicles. Does the story vary much between book and series? I understand plot lines will have been cut but is the overarching story the same?
Season 1 basically covers The Elfstones of Shannara, which is the second volume. What happened in the first volume (The Sword of Shannara) is used as the backstory of Season 1 - and is more or less told in the same way.
Season 1 follows the basic plot of the novel, which several very significant deviations:
- In the books Wil is already studying to become a healer in Storlock at the start
- Bandon & the sword of the warlock lord are completely original to the television series
- The main female characters (Eretria & Amberlee) are both much more action girls in the television series
- The Dagda Mor is not an ex-Druid in the novels
- Eretria does not have an integral connection to the main quest (i.e. is not required to access the Bloodfire) in the novel
- The world being post-apocalyptic is much clearer and more important in the television series. In the novels there are pretty clear clues, but nothing as overt as ruins of San Francisco and working old world weapons
- The television series tries to be a bit darker and grimmer, with more moral ambivalence. For example, the elves did not torture people in the books (the entire Pykon episode).
- Allanon seems more powerful in the books and tends to fight with blue fire (called Druidfire) instead of an extensible sword
What is the same: Ellcrys dying, murder of the Chosen, demon invasion, Amberlee turning into new Ellcrys, Allanon (last druid), Elfstones, Reaper and Shifter demons playing important roles.
Season 2 is more or less completely original. Mareth does not appear in the novels (or to be correctly a more or less unrelated character of that name appears in the prequel novel First King of Shannara). The Warlock Lord never returns. Heaven Well's does not exist in the books. Leah is a very minor and rural kingdom. Some characters are lifted from other books (i.e. Cogline and Garet Jax are both from The Wishsong of Shannara).
Most importantly, in the novels Wil and Eretria did no longer appear (in person) after Elfstones.
Season 2 is a very loose adaptation of Books 3 (The Wishsong of Shannara) and pieces of Book 1 (The Sword of Shannara) of the original Shannara trilogy, with the following peculiarities:
- In Book 1, the Warlock Lord is completely destroyed to never appear again. But in Book 3, the same evil, represented by a sentient magical book Ildatch (never mentioned in the show), spawns monsters and seduces creatures with evil tendencies (mostly Gnomes), intent on possessing the entire realm.
- Bandon line is original to the series. In the books, the evil wakes up itself (Ildatch commands the Gnomes to take it to their land, and then begins building its protective surroundings and spawning evil creatures from other worlds, like Jachyra). This happens slowly over time, until Allanon wakes up, aware of imminent danger, and calls for help.
- Events of book 3 occur some 20 years after book 2, and Wil's children have to stop the evil, with Wil and Eretria completely unaware.
- Heaven's Well appears in Book 3 as the place where the Dark Forces threaten to poison the Silver River (and thus all Four Lands), and is a place that can grant magic powers. It plays a crucial role in defeating the evil.
- Kings and Queens play a minor role in book 3, unlike the series. Also, most of them stay alive.
- General Riga and his minions intent on destroying magic are original to the series. Possibly loosely based on the Seekers of later books of Shannara.
- Family relations: Wil is Shea's grandson, not son, as in the Series. In book 2 (The Elfstones of Shannara), Shea is mentioned to be alive and well, no mention of him in Book 3 (possibly dead of old age).
- In the books, Eretria has no magical powers. She is a regular Rover girl.
- Cogline only appears in the end of Book 3, and is said to be a hermit who never leaves his domain. He has no magical powers, and only uses some explosives. His talents of a scientist are revealed in other books.
- Book 3 events are mostly concentrated in the Eastland, nothing important happens in other lands once two parties of heroes travel there.
To sum up, creators of Season 2 decided to make a spin-off, or a loose adaptation. In my opinion, they failed.
update: To avoid confusion mentioned in comments: The Sword Of Shannara and its sequel novels The Elfstones Of Shannara and The Wishsong Of Shannara are the original Shannara trilogy. The Word/Void series (Running with the Demon, etc), The First King of Shannara and some other prequel books were written and published after it. So book number referencing in my answer relates to the original trilogy only. For more details, one should look for the chronological or suggested reading order, here and in some other places.