THE SHORT SECOND LIFE OF BREE TANNER is an imperfect book. Bree is not a compelling character. She is a device employed by the author to shed more light on the established canon characters, in the guise of new material.
Bree was introduced briefly at the end of ECLIPSE. After the big vampire/werewolf/evil vampire battle, one of Victoria's vampires surrendered and the Cullens briefly entertained the idea of taking her under their wing, and introducing her to their ways.
But then the Volturi came along, and did their rule enforcement thing, and killed the vampire. And that vampire was Bree.
In TSSLoBT, Meyer gives Bree her own book. Well, novella. Basically, Bree is in vampire boot camp, with Riley being as vague as possible, lying to the vampire trainees about everything under the sun (including the sun itself). Bree suspects that Riley is maybe not as shit-free as he claims, but as a fledgling she's pretty dependent on him, and it's a little terrifying to think that your maker might have Sinister Plans for you. Bree also has a sort-of romance with co-trainee-vampire, Diego. Their romance is poorly done and one of the worse parts of the book. BFFs, are you serious? Their banter was painful.
The best part of the book is probably the last thirty pages, which makes me think that this would have worked better as a thirty-page short story that she could have put up for free on her website or something, instead of bloating it out, and trying to make a(n ill-fated) love story out of it.
THE SHORT SECOND LIFE OF BREE TANNER, as others have said, really highlights a lot of the issues with the TWILIGHTverse world-building.
For example, vampires apparently sparkle because the sun reflects off their skin. (But why?)
Vampires have venom instead of blood/saliva/semen. (But then how are babies made?)
Vampire gifts are really rare -- 1/50. Since there are seven Cullens, plus Bella, the probability of them all having special talents is 1/50 x 1/50 x 1/50 x 1/50 x 1/50 x 1/50 x 1/50 (+ Bella = x 1/50).
Do you know what the possibility of that is?
One in seven-hundred and eighty-one billion, two-hundred and fifty million.
Talk about your special snowflakes.
I also found it weird, how vampire "taste" is explained. So Bree and co. feed off "dregs", which are basically prostitutes, pimps, and the homeless (or teen runaways). Apparently, they are all on drugs, so humans who don't do drugs taste better. Which I guess would make sense. But then, Bella apparently smells excellent to all the vampires, which puzzled me. Does that mean that there's something else added to the mix? Because I thought she was supposed to be irresistible only to Edward. What makes her smell so sweet? Is it the fact that she's a virgin? But surely there would be some virgin teenage runaways who don't do drugs.... It's all very confusing, and really says a lot about how Meyer views the lower classes (drug-addled sluts, clearly).
It was weird in general reading a Stephenie Meyer book that was under 500 pages. I actually like her style of writing, and it was nice to revisit the book series. It's sad that she doesn't write anymore. Maybe she's happy she no longer needs to. Maybe she's pissed off that people keep publishing (and cashing in on) fanfiction based on her work -- I know I would find that upsetting, as well.
Overall, THE SHORT SECOND LIFE was a disappointment. Definitely one of her worst books (although a sight better than the monstrosity that was BREAKING DAWN). It passed the evening, and now I think I'll pass the buck (I mean book) along to my sister. See what she thinks.