What is the standard narrative text to read "Tristan and Isolde"? That is, is there something that is a classical text, not in poem form, something like Le Morte d'Arthur for King Arthur?
The versions of the story of Tristan and Isolde that we know today date from the Middle Ages, a period in history when copyright did not yet exist, when originality was not the highest goal in literature, and when imitation and emulation was a perfectly respectable approach to creating new works of literature. This means that medieval literature was based on a different set of concepts than those we value today (especially since the Romantic period). In this context, the earliest version of a specific story is not necessarily the one that is best known today, because later authors may have retold the same story in a "better" way. ("Better" from a more modern point of view, usually). In this context, one text may become more popular than another. But what would make one version the "standard"? The most popular one? That may change over time. The first one? That one may have been forgotten or even lost. For these reasons, talking of a "standard text" of a story that has been retold several times may be questionable.
Chrétien de Troyes (1135?–1185?) seems to have written a work on Tristan and Iseult, but this has not survived. The German poet Eilhart von Oberg(e) (late 12th century) wrote the oldest surviving version of the story entitled Tristrant (around 1170). Eilhart's version was based on a French estoire (story/history), which might actually be Chrétien de Troyes's version. The same source was also used by Bérol's/Béroul's Tristan, written after 1190.
Thomas d’Angleterre (Thomas of Britain) also wrote a version of the story around 1160–1165; this version has come down as eight fragments. This version is older than those by Eilhart von Oberg(e) and Bérol/Béroul, but also very incomplete (i.e. much more fragmentary than the other versions; see for example the fragments on Wikisource). Gottfried von Strassburg (who died around 1210) wrote what is probably the most influential version, Tristan, which is considered one of the masterpieces of German medieval literature. (See the Middle High German version of Tristan, Karl Simrock's translation (1855) and Hermann Kurz's translation from 1877). Richard Wagner used it as a source for his opera Tristan und Isolde. However, even this version was left unfinished. Ulrich von Türheim (around 1240) and Heinrich von Freiberg (around 1290) provided endings or continuations.
There is also a 13th-century French Prose Tristan, which is currently only available in a nine-volume scholarly edition (Le roman de Tristan en prose). An English translation by Renée L. Curtis published by Oxford University Press in 1994 is now out of print.
Most prose versions of the story are of a much later date. Le roman de Tristan et Iseut (1900) by Joseph Bédier used Béroul's version as a source of inspiration, whereas earlier authors had preferred Gottfried von Strassburg. Rosemary Sutcliff published a children's novel based on the Tristan and Iseult legend: Tristan and Iseult (1971), which was well received (Boston-Globe Horn Book Award in 1972). The German author Günter de Bruyn published a retelling in 1975: Tristan und Isolde. Dieter Kühn retold or translated Gottfried von Straßburg's version in 1991: Tristan und Isolde des Gottfried von Straßburg.
Conclusion: Based on Elisabeth Frenzel's discussion of the material, the most influential medieval versions that have survived are those by Béroul and Gottfried von Strassburg, both of which are in verse. None of the prose versions have had a similar impact. Based on this there is no medieval prose version that can be considered a "standard", even if we allowed that talking of a "standard" made sense in this context.
- Frenzel, Elisabeth: Stoffe der Weltliteratur. Ein Lexikon dichtungsgeschichtlicher Längsschnitte. 10th revised edition. Stuttgart: Kröner, 2005. (Pages 919–927.)
- Le roman de Tristan en prose. Edited by Philippe MÉNARD. Librairie Droz. In nine volumes. (See e.g. volume 1: Tome I, Des aventures de Lancelot à la fin de la "Folie Tristan", 1987; volume 9: Tome IX, La fin des aventures de Tristan et de Galaad, 1997.)