If they play video games,
Kaiserreich: Legacy of the Weltkrieg is by far the best mod for Hearts of Iron, the best content available (paid or otherwise) for that game, and a fantasically detailed AH, improving in terms of plausibility with every update (while keeping most of the fun).
Eric Flint's
1632 and its many sequels are a great place to start for ISOTs. They start to decrease slightly in quality around 1635 and are slowly recovering, though. That's my opinion anyway.
ISOT proper, I like the concept but have problems with Stirling's, um, attitudes that come out in the writing. It's also more pulpy than most traditional AH and ISOT works, in a way that can be irritating to some readers.
Don't start with the Draka. Just don't.
The Two Georges and
TL-191 are both decent but flawed options, but skip the post-Great War books from TL-191. That's when it straight-up jumps the shark.
Good timelines from AH.com are also IMO good ideas.
What Madness Is This? (currently in its second iteration),
Look To The West, AANW, and the ongoing "1945 US ISOT to the Two Georges" thread down in ASB that
@MikeTurcotte is going, those are all great options IMO.