The tale of Tristan And Isolde is a classic legend, most famously retold by Richard Wagner in his music drama. It is the classic tale of passion vs. duty, and as long as society demands that we subjugate our passions to it's moral and legal demands, as long as civilization persists, it will continue to have it's relevance.
In 1900 Frenchman Joseph Bédier, probably spurred by Wagner's revival of the tale, published his retelling in prose of the tale, a synthesis of the medieval sources such as those of Béroul, Thomas, Eilhart and Gottfried.
It's a brilliant work, making the tale accessible and enjoyable to modern readers. The medieval sources can be hard to find in translation and are probably just as hard to understand. The story does however remain very much relevant and Bédier doesn't need to change it, only re-tell it. And that he does well.
His style is interesting. He avoids the style of the modern novel, with it's close accounting of events and thoughts, instead going for a conversational style that reminds me of medieval epic poetry I have read, yet fully modern. It reads like a transcription of a masterful oral re-telling, full of asides and comments to the audience, that does not take one out of the story and instead enhances it.
To illustrate here's the beautiful first paragraph in Hilaire Belloc's English translation:
"My lords, if you would hear a high tale of love and of death, here is that
of Tristan and Queen Iseult; how to their full joy, but to their sorrow
also, they loved each other, and how at last they died of that love
together upon one day; she by him and he by her." Notice how he uses "hear" instead of "read".
This is highly recommended. While not the innovative masterpiece of Wagner, it is probably the best way to introduce yourself to this legend.