The ascendancy of tablets (like the ascendancy of smartphones before it) is to a great extent attributable to their affordability which itself is fostered by an expectation by tablet manufacturers that purchasers will consume content from a specific consumption space: iPad = iTunes Store; Kindle = Amazon.com; other Android = Google Play Store; Windows Phone = Microsoft Store. So the answer to your question is very much dependent on which consumption space works best for your brother-in-law. The Kindle Fire HD would be a good choice for someone who is likely to capitalize on Amazon's books, music and video offerings.
We were recently in the position of deciding between the Kindle Fire and the Google Nexus. We're Amazon Prime members so it seemed, at least initially, that our choice was clear. However, we actually ended up going with the Nexus. This was mostly a matter of how much more limited the Amazon consumption space was with regard to apps, specifically (since [a] we consume Amazon.com books on our Kindle Paperwhites, and prefer that to reading books on a tablet; we are already heavily invested in Zune for music; and [c] we don't enjoy watching video on small form factor devices).
Our biggest concern was availability of apps for the fundamental aspects of our day-to-day online experience: Email through Outlook.com; SkyDrive; OneNote; etc. Our earlier smartphone choices (driven by employee discounts we had at the time) have tied us very much to the Microsoft consumption space, but we definitely didn't want a Windows tablet. At the time we made our choice, Google Nexus provided a better bridge between these Microsoft environments than Kindle Fire. I don't know if that's still the case.
So how does this inform your choice for your brother-in-law? To a great extent, only from the standpoint of pointing out how "it depends" and pointing out how things change over time.
I will inject my standard warning against thinking that a tablet is the way to go for reading books. There's a reason why e-ink is preferred for e-readers. I would recommend buying a lesser tablet to save $120, and use that $120 to buy a Kindle Paperwhite for reading, rather than trying to solve the need for books with a regular tablet.
We bought a 7" Nexus (for my spouse) and a 10" Nexus (for me). The 10" is "too big" for most things that most people use a tablet for. I use it as a mini-workstation, accessing my email and taking notes, at business meetings. (I even have a portable, Bluetooth keyboard for it.) I use the tablet as my script when I'm a worship leader or worship associate at church, instead of a binder full of papers. It does those things better than a 7" could. But for reading news in a train terminal or airport; for driving directions in the car; etc., the 7" is better.
I'm not sure if the Kindle Fire's 8.9" is "just right" though. I think the handy convenience of the 7" form factor cannot be overstated. But if your brother-in-law is going to be using the tablet mostly in a truck cab (presumably stopped in a truck stop) then bigger is probably better. And if he's got any concern about the visibility of small type, then a 10" tablet is worth considering. There's no way around it: My 10" Nexus is the most readable tablet there is, as far as I'm concerned.
