Useful? Yes, but at 48 pages total (inclusive of cover pages, table of contents, all the filler/spacer pages), it's not the kind of PDF that says, "I'm overflowing with stuff you don't need."

About half the book is dedicated to personas from the comics, including the 4 Grey sisters... so if you're shying away from the comics, this might not be that useful. If you're looking for a condensed read of the characters in one spot, then this might be what you're looking for. And it includes very key characters such as: The Kaiser Hind von Medeim, Queen Alena Zakarhin von Medeim (don't ask me why she's a "Queen" and not "Kaiserin"), The Lord High Marshal/Wolf General, and other characters from the comics.

If you're looking to add these figures into your stories, then you're in luck. Half the PDF is dedicated to them. The background information is probably of more use than the stats, for the most part. d6 never struck me as a game system where you hunt down the biggest enemies possible to compare stats (unlike Dungeons & Dragons).

The rest of the book?

About 10 pages of additional information on the setting, which is basically more regional information that is very very similar to the layout/presentation of the main powers in the Carbon Grey RPG book. Don't expect a lot of excess description. It's a couple pages at most on these countries, like the Turkul Empire (i.e. the Ottoman Turks of the setting). Having the info is great, especially if you want your game to detour into the Ottoman/Turkul Empire... but it's brief. It's enough to create something from, but don't expect extensive backgrounds and maps and other details.

The third part (`8 pages) is a section of general NPC stats, again similar to what you have in the main Carbon Grey RPG rulebook... but simply more of them. More stat blocks is goood. The stat blocks give you general ideas of who they are for your setting... so "Rival, Solo" Sniper instantly tells you what kind of use this stat block is for, just like the stat blocks in the main rulebook. "Rival Leader" Purge Officer, "Solo, Rival" Hitman, "Solo, Rival" Manipulative Elite, and "Solo, Rival" Peerless Ace give you some ideas of what to expect from this section. This is definitely very useful reference material.

Last is a single page with a set of 6 random Espionage adventure hooks (i.e. roll d6, get a hook). Useful but brief.

Overall, you can say this book is "useful but brief". Half of the book is stats I don't really need, but the character info is actually quite useful in fleshing out the world even if you don't intend to use the NPCs like the Kaiser in actual play. It's good setting information, but through a character lens.

The setting info about factions & world powers? It's merely okay. It doesn't expand on the formula in the main rulebook, which is very brief and gives you enough to get a sense of the world... but making the world is left up to you. (If you like the loose setting in the main book, then this is fine. If you need more direction, then don't expect it here.)

The most useful section, with stat blocks for various general NPC types? It's the shortest section of the book. I find it very useful, but it's brief.

Overall, a good book. You'll want to get it if you're interested in this game... but it's not jam-packed with stuff. It takes a clever GM to make maximum use of most of the book outside of the generic NPC stat blocks & the one Espionage table.