These helpful responses are much appreciated.
Hmm! Sounds to me like "small is good", and may make me think about organising the books in the best way, rather than dumping the whole lot on at once.
There are certain books I will want to keep on the device permanently but
most of them can be deleted once read. I have all the ebooks (doubly) backed up anyway: on my computer and on an SD card. So if I ever yearn to read them again, it is not a problem.
So, I'm now thinking, it may make better sense for me to upload maybe 6-10 at a time, read, delete and then upload the next small batch. Does anyone follow this approach?