Some DVDs, or just some parts of some DVDs, have an attempt at encryption. Of course this is the kind of pointless self-contradictory encryption where the person who should be able to read the content is the same as the person who should not be able to read it, so it is easily bypassed.
It seems that the easiest way to do this is to build libdvdcss from source, and then to install the Debian packages of libdvdread (which detects libdvdcss at runtime) and vobcopy (which provides a command-line tool to hide all the complexities). It's then possible to mount the dvd and to copy the contents as follows:
# mount /dev/sr0 /mnt/dvd # cd /path/to/destination_directory # vobcopy [-n <title-num>]
For a film there may be only one "title" worth copying, and vobcopy will probably select it automatically. If there are multiple "titles", i.e. episodes of a TV series, you can specify them with -n. vobcopy -I will show useful information about what's present.
Using vobcopy, I've read a couple of DVDs and can play them. I don't yet have a program that will play the data directly from the DVD, and I may not bother: the DVD mechanism is quite noisy, and it may be preferable to copy the disks first and then play them silently from the SSD.