Hortulus Animae (or better in French) is the usual answer. See, for instance, section 3 of this essay:
Just as one need not know precisely which "certain German book" Poe is referring to in order to proceed to the second paragraph and beyond—again, we do not learn the identity of this book until the story's last sentence—so one does not need to be able to decipher every word or phrase to make headway with the tale.
I do not pretend to understand this.
The phrase that Poe quotes, "er lasst [or "lässt] sich nicht lesen" does not seem to appear in the text, at least my brief skim through the digital image of a copy at Heidelberg, nor is it a phrase that would be expected to appear. This phrase, which the German-to-English part of Bailey-Fahrenkrüger's Wörterbuch der englischen Sprache (1822 edition, p.467) glosses as "it does not bear reading", shows up idiomatically in pre-1839 German literature.
A later printing of the Hortulus is described in Isaac D'Israeli's Curiosities of Literature in these terms, in his section "Religious Nouvellettes":
I shall notice a class of very singular works, in which the spirit of romance has been called in to render religion more attractive to certain heated imaginations.
In the fifteenth century was published a little book of prayers, accompanied by figures, both of a very uncommon nature for a religious publication. It is entitled Hortulus Animæ, cum Oratiunculis aliquibus superadditis quæ in prioribus Libris non habentur.
It is a small octavo en lettres gothiques, printed by John Grunninger, 1500. "A garden," says the author, "which abounds with flowers for the pleasure of the soul;" but they are full of poison. In spite of his fine promises, the chief part of these meditations are as puerile as they are superstitious. This we might excuse, because the ignorance and superstition of the times allowed such things: but the figures which accompany this work are to be condemned in all ages; one represents Saint Ursula and some of her eleven thousand virgins, with all the licentious inventions of an Aretine.
and much more of a similar kidney. D'Israeli does not approve, and might well have written that this book ought not be read. (The possibility that Poe knew of Hortulus through D'Israeli is given in a footnote in the Library of America edition of Poe, edited by Patrick F. Quinn. The form of Poe's footnote citing the Hortulus is
{*1} The “Hortulus Animæ cum Oratiunculis Aliquibus Superadditis” of Grünninger
which is plausibly a shortened form of D'Israeli's citation.) The phrase sich nicht lesen does not appear in D'Israeli's book. Were there German editions of Curiosities, or German analogues of Curiosities, in which Poe might have seen the phrase in question?
Added 21 December. Among the German disparagers of the Hortulus was Martin Luther, whose own Betbüchlin [sic] (of 1522) or Personal Prayer Book starts with this negative assessment:
Among the many harmful books and doctrines which are misleading and deceiving Christians and give rise to countless false beliefs, I regard the personal prayer books as by no means the least objectionable. They drub into the minds of simple people such a wretched counting up of sins and going to confession, such un-Christian tom-foolery about prayers to God and his saints! Moreover, these books are puffed up with promises of indulgences and come out with decorations in red ink and pretty titles; one is called Hortulus animae, and another Paradisus animae, and so on. These books need a basic and thorough reformulation if not total extermination. And I would also make the same judgment...
Various old editions are online; I have looked at the 1544 Wittenburg edition, and have checked the above translation, which I cribbed from p 231,232 of "Luther on the Psalter" by C. A. Aurelius in T. J. Wengert Harvesting Martin Luther's Reflections on Theology, Ethics, and the Church.
Wouldn't it be cool if Poe's source for "er lässt sich nicht lesen" was some other piece of writing by Luther?