Schubert set texts by no less than one hundred and eighteen poets, including one poem he wrote himself. Goethe (74), Schiller (44) and Mayrhofer (47) were his staples, and he returned to these poets time after time (Mayrhofer was for a period a close friend). Others played an important rôle for a time, and were then forgotten: Kosegarten, Matthisson and Hölty come to mind. Some were friends, like Bruchmann, Senn and (in the last years of his life) Eduard von Bauernfeld, who was the only one brave enough to visit the typhus-ridden Schubert on his deathbed.
Having a poet friend is handy, but Schubert's thirst for new texts could lead him to make settings of artistically dubious works such as Jozef Kenners Der Liedler, the only song in the whole Lieder repertoire to feature a werewolf. (As a caveat I should point out that precisely that song and the poem that inspired it were immensely popular at the time). Many of Schubert's most popular, even greatest songs were settings of comparative non-entities, for instance the (confusingly named) Christian Friedrich Daniel Schubart, who provided the texts for Die Forelle, or Friedrich Leopold, Graf zu Stolberg-Stolberg, who wrote poetry every bit as clumsy as his name, but those listening to Schubert's setting of Auf dem Wasser zu singen are not likely to complain. (Incidentally, the better the poetry the more easy it is to memorize, in my opinion. Auf dem Wasser zu singen I find a real challenge in the memory stakes, as I do Die junge Nonne, feeble indeed as verse but a candidate for his best song.
Quite a few settings are translations of foreign poets: Shakespeare, Scott, Pope, Ossian (the wily James MacPherson and his bardic "discoveries", rumbled by Dr. Johnson), Petrarch and Metastasio. The texts in English were all translated into German, by Herder among others, the Shakespeare settings by his friend Eduard von Bauernfeld, editor of the Vienna "Shakespeare Ausgabe" of 1825, and himself the author of Der Vater mit dem Kind, a personal favourite. The Italian settings, with the exception of Petrarch, were composed to the original Italian texts, although in the case of the best of the Italian songs, the "Drei Gesänge für Bass-Stimme" Schubert provided German words. Salieri, Schubert's composition teacher, encouraged the young composer to write in Italian, which among other things might explain the italianate, proto-bel canto style one finds in many of the lyrical settings such as "Heimliches Lieben" or passages in "Die abgeblühte Linde", not to mention the searingly beautiful cantilena in "Ellens dritter Gesang" (Ave Maria): both Schubert and Salieri greatly admired Gluck, Finally, there are a no less than eighteen settings of Anonymous (some of which have recently been attributed). Some of them may well have been penned by Schubert himself.
Illustration (untitled) by Elisabeth Rivers