I conducted a bit more research on the topic and found that such items were used primarily as medical references, astrological calendars, and more rarely for any other uses requiring a pocket reference (eg psalms, canon lore, etc). They go by various names: Vade Mecum, folding almanac, folding calendar, and a few other rarer descriptions - no single designator for these exists, although Vade Mecum comes closest, and a variety of terms are used, such that sometimes it is not possible to determine without a photo or very detailed description if an item is such a folding book or a simply a very small bound pocket book.
A variety of folding patterns were used. Here is an examples on the Internet (unfortunately not showing how it folds) and another example which is closer to the folding pattern I saw in the state library, and finally an example with multiple views showing how it folds.
I wish to write about these more, but my notes are not in order yet. Meanwhile, I've been experimenting with making quick and simple mockups of these in paper, and I've had a request to share the process of making these.
The aim was not to make a high authenticity copy of these medieval folded reference books, but to make a cheap and easy replacement for the kind of reference materials that people often carry around at events - notebooks, timetables, class descriptions. Made from paper these would not be as durable as from parchment, but they look a lot better than a sheet of white photocopied paper, and do work quite nicely in a pouch, and presumably on your belt. (I don't frequently wear a belt, except with outer garments - often in damp or darkness- so I haven't tested this yet)
A sheet of A4 paper actually makes a nice size for turning into a folding book, although given the diversity of styles used, other sizes would work too (letter is similar enough in dimensions to use in this way). I used A4 sheets of 200gsm fake parchment card - that's about twice the thickness of photocopy paper. But photocopy paper will work fine to test out the shape you want to make. The originals would have been in vellum, which would be much more durable than paper, but as we don't expect to make these last for decades, the paper will last long enough. I advise using a wooden or stiff plastic ruler or paper folding tool to carefully press the edges of your fold below. Use some care in how you fold the paper if you want a nice symmetrical end product.
Fold you sheet of paper in half along the long edge:
Leaving your sheet folded in half (unlike the above picture), now fold the paper in half along the new longest edge (in the opposite direction) and unfold. Then fold the sides in towards this centre line to create two more folds parallel to this centre fold:
Unfold all fold except the first large one. Fold down a line parallel to this 1st fold, approximately 2cm down the page (the exact distance doesn't matter, but it must be the same for all pages so choose a distance and measure it).
Open out your paper and it should look like this:
Now, cut out a section from the center of the page , cutting along your last folded line, and leaving one quarter of the centre section uncut, like this:
Refold your pages into their final arrangement (this should be simply following the already made fold lines), first one side:
then fold the other side too:
You have one completed pair of pages. Make some more:
When you have a number of pages you are happy with (I recommend using less than 5 pairs on your first try - the paper thickness gets a bit tricky), line them up together with the pages all opening in the same direction (bulldog clips help at this stage):
For a quick folding book, I stapled the top sections together at this stage. Medieval books would have been stitched, or I could have glued this, but I was aiming for speed and this section would be covered. I also punched holes in this section at this stage with a hole punch (again, speed over accuracy). (sorry there's only a blurry picture)
Next, I took some of my trimmed off pieces of paper and glued this over the staples on the front side of the book (bulldog clips help):
When the glue was dry I folded in the edges of this snippet, so they enclosed the pile of pages, then folded over the top to hide the seams, and glued all of this. Finally, when this was dry, I punched through the hole in the top in the same place it was in the pages underneath. A finished quick and cheaty folding book:
You can fairly easily setup a word processor to print to these pages, or make a blank one to use as a notebook (not a period usage, but more authentic feeling than a modern notebook if you're not quite ready for a wax tablet). In period examples, sometimes the folds are used as dividing lines for columns, and sometimes they are completely ignored - often in the same document, or even page - so don't feel too confined in your manner of writing.