I have a Likebook Mars e-reader. When I plug it into my laptop with a USB cable, it appears in PCManFM-Qt, and I can access the files on it graphically from there:

That works okay. What I want to do is browse, read, and write files on the command line with cd, ls, cp, etc. But the e-reader doesn't seem to be reachable via anywhere I can cd to in the real filesystem (even if I've already looked at some of the files in PCManFM-Qt, which in other cases seems to be necessary). Having looked at the question How to access a usb flash drive from the terminal?, I tried lsblk, but the e-reader doesn't seem to be listed:
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
nvme0n1     259:0    0 953.9G  0 disk 
├─nvme0n1p1 259:1    0   300M  0 part /boot/efi
├─nvme0n1p2 259:2    0 653.6G  0 part /
├─nvme0n1p3 259:3    0    16M  0 part 
└─nvme0n1p4 259:4    0   300G  0 part /media/hippo/A4A60CCCA60CA142
df provides no hints, either:
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           1.6G  1.9M  1.6G   1% /run
/dev/nvme0n1p2  643G  240G  371G  40% /
tmpfs           7.6G     0  7.6G   0% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
/dev/nvme0n1p1  300M   33M  267M  11% /boot/efi
tmpfs           1.6G   92K  1.6G   1% /run/user/1000
/dev/nvme0n1p4  300G  154G  147G  52% /media/hippo/A4A60CCCA60CA142