Help with a travel guide for xmas gift?

[autismmumwithafamily]

Hello,
What is the best travel guide for my BIL & SIL. They are retiring from the police next year (so they are still youngish!) and are going to travel in a mobile home now their kids are grown (cool & jealous..I know!!) can anyone recommend a good book as a xmas gift please. They aren’t planning any particular countries yet (have mentioned USA, Australia & Europe..)
Thanks MNr’s xxxx

[tripfiction]

I quite like Marco Polo guides - they don't cover all destinations... Horizon guides are different. Otherwise DK Eyewitness. Hope they have fun!