Darn, just managed to lose my original post.
However, reading what you just said, I shall now say "Get another shop".
In brief, what I had originally written down included Neal Stephenson (
Snow Crash,
Cryptonomic,
Diamond Age, etc). "The Difference Engine" (Sterling? Gibson? Both of them? Wiki it, to make sure) is an exemplar of its kind. I was also seconding KSR's
Red/Green/Blue Mars trilogy. Though none of them recent, Asimov and Clarke (you already mentioned the lack of Rama-ishness, however) are always good. Either full stories of collections. More modernly, again, Stephen Baxter. Oh dear, quite a lot of A, B and C names come to mind. Probably because of my tendency to read through the library SF/Fantasy section in alphabetical order, three or four decades ago.
Oh, and
do try
Wyrd Sisters, if that's the only Pratchett they have (another good sign that it's not a good source for books is if they
don't have most of the dozens of Discworlds/etc that he has written...). It 'starts' (almost, there's
Equal Rites before it, but there are character differences in the still-developing world being represented) the "Witches" arc of the Discworld books, which you can follow if you get the taste. But if you can find
Guards! Guards! to start with, that might be a better introductory one (unless and until you become a 'purist' and start with
Colour of Magic/
The Light Fantastic, as first two books,
Equal Rites being the third). Google for "Discworld Reading Guide" for some (not necessarily up-to-date, but still quite populous) visual explanations of the various arc connections.
There are also non-Discworld books you might like from the hand of the aforementioned author. Might I suggest
Nation?
Good Omens (Pratchett and Gaiman, for the latter of which I'd suggest Neverwhere in one its various formats, text-only or graphical novel) is also good, and
should be available.
Sorry, writing too much. And reading too little. Just today I got the latest
Science Of Discworld, which I really need to get back to reading. Coincidentally, I also got Ringworld (as a birthday present) and I'm wondering whether to risk its spine by re-reading it, having never had hands on a non-library copy before, and that way too long ago.