Dos-à-dos binding refers to a technique whereby two books are bound at their backs, allowing you to read one book, flip the conjoined pair over, and continue reading the other book.
The dos-à-dos book you see here is even more special. Not only is it a rather old one (it was bound in the late 16th century), but it contains not two but six books, all neatly hidden inside a single binding (see this motionless pic to admire it). They are all devotional texts printed in Germany during the 1550s and 1570s (including Martin Luther, Der kleine Catechismus) and each one is closed with its own tiny clasp. While it may have been difficult to keep track of a particular text’s location, a book you can open in six different ways is quite the display of craftsmanship. Edited from io9