How can Irish makes the distinction between "what" and "which"? It does not always matter in English, but occasionally the more accurate "which" is better. For example, it doesn't particularly matter if I ask "What bus do you catch home?" or "Which bus do you catch home?"; but if I am looking at a tray of different varieties of ice cream, I might ask myself "Which (one) shall I have?" but not "What (one) shall I have?"
I thought Cé acu leabhar a léann sí? but I am a bit thrown by the singular leabhar. Can anyone help with this conundrum?
What makes you think that there's a difference between "what bus do you catch home?" (which you find acceptable) and "what one shall I have?" (which you find unacceptable)?
If instead of "which one shall I have?" you had said "which of them shall I have?", then swapping "what" for "which" is indeed a problem, and so cé acu is closer to "which of them".
Your concern with the singular leabhar is probably due to the confusion in English between the singular "which book" and the plural "which (one) of the books".
Note also that cé acu is used for "whether" - cé acu is maith leat é nó nach maith - "whether you like it or not".
It doesn't "work better" in English, because it means something else. Irish and English both differentiate between the simple present and the present continuous, though not all European languages make that distinction.
The Irish for "what book is she reading?" is cén leabhar atá sí a léamh?