The Torchwood Archives is a fun "in-universe" companion book to the show. The premise is that it's a compilation of material cobbled together by an author named Warren Martyn who got so close to the truth that his manuscript had to be seized and placed securely in Torchwood's vaults. The book is cleverly done to look like the real thing, complete with faux-coffee stains, "taped-in" photographs, post-it notes, handwritten marginalia, etc. Functionally, it contains some interesting history of the Torchwood Institute prior to Season One and then serves as a bit of an episode guide for the first two seasons. Here's what it contains in more depth:
(1) A fun preface by Ianto explaining how the book came into Torchwood's hands and who Warren Martyn is/was.
(2) An introductory letter from Warren Martyn to his planned publisher, explaining the basis for his strong belief that Torchwood exists. The letter references all of the craziness that London has experienced from fifty+ years of Doctor Who to show that, obviously, aliens are among us.
(3) The Torchwood Charter from 1879 assented to by Queen Victoria. It establishes Torchwood's primary purpose (protecting England from visitors from other "heavenly bodies") and sets forth the role of the Torchwood Institute (a.k.a., Torchwood One in London), Torchwood Glasgow (Torchwood Two), and Torchwood Three (Cardiff--the one we know!).
(4) Notes on Torchwood One and Torchwood Two. Includes a lot of speculation about what happened on Canary Wharf.
(5) Notes on Torchwood Three. There's a ton of really interesting little bits and pieces here, including:
--speculation that there are underground tunnels connecting the Torchwood sites;
--mention of an earthquake destroying part of Torchwood Three in 1906;
--a "stolen e-mail" from Tosh to Suzie Costello explain her theories on the rift;
--an e-mail from Suzie wondering why Torchwood didn't intervene in the near-explosion of a nuclear plant in Cardiff (from Doctor Who's "Boom Town" episode);
--speculation about Jack, Suzie, Owen, Tosh, Ianto, and Gwen (interesting side note: Jack took over leadership of Torchwood in 2000 after the New Year's Eve massacre; Tosh joined in 2005, Owen in 2006, and Gwen & Ianto joined in 2007; when did Suzie join and who else, if anyone, was on the team in the years 2000-2004?)
(6) Episode-by-episode notes. It's a bit weird to put episode titles in a book that's supposed to be "in-universe", but perhaps we can fan-wank this as the titles given to each incident by Warren Martyn. There's various in-universe memos, police reports, photos, etc. about each incident. My favorite bits here are memos written by Jack that subtly show his and Ianto's growing involvement and a memo from "1999" by James Lawson to Alex Hopkins on the development of RetCon (James Lawson is Owen's predecessor as team doctor).
The Torchwood Archives is obviously pre-Children of Earth, though it does include some material from audiobooks like Lost Souls. Overall, I thought it was clever and well-done. Sadly, Ianto notes in an epilogue that author Warren Martyn is missing, so surely there will never be a sequel . . .