I've expended a great deal of energy finding copies of books with made up, encoded, or
asemic writing; more than seems proper. Hopefully by compiling the documents here, I can make it a little easier for everyone else.
Most files are already available online as PDFs, so I've just linked to those, and others I compiled and put on my google docs account for download. They are roughly in order of how much I mentally categorize them in with the Voynich Manuscript.
Let me know if anything is wrong, missing, broken, or otherwise problematic.
The Voynich Manuscript

This is the first document that caught my interest; it was discovered in 1912 by a book dealer, Wilfrid Voynich, and consists of over two hundred pages of indecipherable (and quite possibly asemic) text and images. There is plenty
more information about the document all over the internet.
Voynich Manuscript (pdf, 56.2MB), originally found here.
Spurious or unverified translations have been proposed by
William Newbold,
Leo Levitov,
John Stojko,
Joseph Feely,
P. Han,
Claude Martin,
Richard Rogers, and
James E. Finn. There are others, but these are the ones I could tie to specific documents.
Codex Seraphinianus
A friend introduced me to this one - it is a more recent (late 1970s, early 1980s) document. It is similar to Voynich in the sense that it records an imaginary world in an invented language and script, but tidier, more organized, and more accessible to the modern reader.
Rumor has it that the page numbering system is base 21.
The Rohonc Codex
The Rohonc (or Rohonczi) Codex is a Hungarian document of mysterious origins and content. It is thought to be written in some sort of proto-Hungarian language, or, more commonly, a hoax of a proto-Hungarian language, rather than an invented world. Nonetheless, as far as I can tell, no one is entirely certain.
Rohonc Codex (28.5MB), compiled from GIFs of each page.
Fair warning, I suspect that a few pages may be missing from my PDF (and, possibly, the original GIFs).
The Dresden Codex
This is a pre-Columbian Mayan codex, and the most complete of the four remaining American documents. While it isn't incomprehensible, fantastical, or asemic, it is beautiful and historical. The Mayans (and, for that matter, all the pre-Columbian Mesoamerican civilizations) are fascinating, technologically advanced, and not given half as much consideration as they should in world history classes. The Dresden Codex is, apparently, an almanac full of highly accurate astronomical information.
The codex is hosted by
FAMSI (Foundation for the Advancement of Mesoamerican Studies, Inc.), along with the
Madrid (48.8MB), Paris (2.9MB), and Grolier (1.5MB) Codices.
Zr + 4HCl → ZrCl4 + 2H2
U + 3F2 → UF6
Roberto Altmann, a Cuban
Lettriste, made this beautiful comic around 1970. It is 13 pages long. That is pretty much all that I know. I assume the dialog is asemic, but don't quote me on that.
Steganographia
I am reluctant to open the Grimoires and Esoteric box because 1) it isn't really
all that relevant, 2) there is just way too much out there, and 3) and Joseph H. Peterson has already made a
wonderful online catalog so there isn't much point in me doing it as well, but I'll include Johannes Trithemius'
Steganographia here because it is, as the name suggests, steganography.
Steganographia (201KB), an html/text version is here.
Fair warning, it's in Latin. Sorry. It may be more interesting to look at the encrypted content than the original document. Ask google.
There are, of course, plenty more documents that I don't know about or can't find enough information on. But hopefully this can remedied.