Thanks for your reply Shaggy.
I've also seen they (the developers) are making efforts to overcome this problem. I've also considered creating a directory structure, but this is kind of limiting, because a particular publication can always be considered to be part of (at least) two classes. For example, a paper on (let's say) "a mathematical solution to the KdV equation via special functions" can be thought of belonging to the 'mathematical methods' category, but also to 'fluid dynamics'. In fact, that's why they invented tags!
Please, see my recent thread on an idea of solving this via an html index file:
https://www.mobileread.com/forums/showthread.php?t=33486
Thanks again for your help & bear in mind:
"Nothing is more difficult, and therefore more precious, than to be able to decide."
Napoleon Bonaparte