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I like how Xanathar's Guide is currently in third place. When I found this book, I was excited to see the new variations and additional clarifications to the rules that I found on there. It is such a great addition to what we currently have. Glad it's on the vote!
I'd like to mention that most of the books for 5e are discounted in the DNDBeyond online store. And if you pay for the top tier subscription you can share it with all your friends and they won't have to pay a dime. Or you could all chip in.
Just a quick note for those interested in DnD: Most of the content (minus art) in the Player's handbooks (and even much of the monster manual) is now covered under the Open Gaming License. This means you can find the info FOR FREE on sites like and plenty of others. It would not make a lot of sense to sell it on Massdrop unless part of a much larger bundle. The other books like Volo's and Xanthaar's cannot be obtained (legally) via the internet. However, they are worth buying if you like the play the game--they contain lots of fun content for players and DMs. A bundle of these books would be an incredible buy if MassDrop could get the at a discount.
I'm surprised that SCAG is dead last - in terms of useful /necessary information for character creation (new domains/orders/paths/etc, race info in relation to the world) it comes second only to Xanathar's IMO. Granted, SCAG is also about 50 metric tons of fluff, which isn't everyone's thing, and Xanathar's adds /SO/ much more in comparison - just wasn't expecting it to be bottom of the barrel.
SCAG (to me) is more useful as a DM tool
Xanathar's has some of the same sub classes, plus a TON of character creation tools
I completely agree - from the stance of character creation, Xanathar's is infinitely more useful - I personally voted for it over SCAG. I was more getting at the idea that one would assume that there are more players than DMs out there, and that it may follow that books that deal with character creation - even if only tangentially, as is the case with SCAG - might be a bit more popular than those that are strictly geared for the DM, like Volo's.
On the other hand, though, I've never DMed a game and I acquired Volo's quite a bit before SCAG, if I recall correctly ¯\_(ツ)_/¯