Morning all. Perhaps a useful pointer is that the endgame regarding the best-selling work is similar to that of Ifor's IQ 1697. It's blogged in detail on Fifteen Squared if you missed that one. The across message is a refinement; if you know what the cells represent, the last word of the message is the key.
The thematic person was born on 15th June, 100 years ago.
Hope this helps - it's a bit vague, I know!