Absolutely not for the faint of heart. There are graphic scenes of torture, and even worse than the ordeal Hanna suffers in body is the torture of her mind. No reader with any actual sense of empathy (real empathy, not immature narcissists demanding we worship their trauma) will be unaffected. However nothing is gratuitous or salacious. It's all in service to the complex and emotional story the author is telling. If you can get past that hurdle and make it to the finale, the reader is rewarded with one of the best descriptions ever put to type of how it feels for a human to experience the mind of an alien species and a truly alien perspective.
Human beings, particularly in science fiction, want to fling ourselves into the cosmos without a clue of what might actually be out there. We just assume we'll solve all the problems that our species will confront with wit, aplomb and technical know-how. Modern-day shows like Star Trek Discovery insist that our adherence and obedience to correct "human" values will overcome the horrors of outer space and the monstrous natures of unenlightened species and people wearing MAGA hats. The truth is that understanding what is alien to our perspective - even the perspective of other human beings - takes work, is painful, slow, and doesn't always turn out well. Lady Hanna of the D'Neera doesn't become one of the most underrated heroines in all of science fiction because she's a telepath and is uniquely suited to encounter a hostile alien form; it's because of her inner strength, her intelligence, her utter refusal to break, and her compassion, as well as her willingness to do the work and never take the easy way out.
A hidden classic of SF. This novel and its sequel, "Master of Chaos", are collected in an omnibus edition called "The D'Neeran Factor", though without the gorgeous James Gurney cover.