(I've already posted a slightly different version of this review on the product page on drivethru rpg. I don't really have time to do the in-depth review of all things Marvels and Prodigies I'd love to do now, so I'm re-posting my review here. Let's just say it struck a note with me!)
I thought I'd never touch another RPG going to the Lovecraft. Well, I made an exception for Marvels and Prodigies, which, I'd dare say - is it really possible? - makes Lovecraftian Horror feel fresh again. It is not simply another "take" on the same endlessly regurgigated Mythos tropes "(It's Cthulhu, but this time with aether-powered biplanes!"); it is an unassuming RPG that has a point of view on cosmic horror that is subtly, but significantly different from "RPG mainstream Yog-Sothothry"; and it has me as excited as back then when I first laid my hands on "Call of Cthulhu" by Chaosium, which was just as slim and unassuming and clearly knowing what it was doing.
The introduction of M&P talks about the ambivalence of xenophobia
and xenophilia that runs through Lovecraft's work and that has been
critically explored by several current authors who write tales of cosmic
horror - specifically, Ruthanna Emrys is mentioned, but I could easily
see M&P emulating the athmosphere and themes of stories by a lot of
current horror authors, Lovecraftian or not, like Laird Barron, John
Langan or Livia Llewellyn. It's about Seekers - people who can't let the
unknown alone, to whom the terrors they encounter always hold the
promise of wonders and epiphanies. It's not a "Let's play the cultists!"
take, but more like: Let's play people who are curious and tough enough
to dig deeper when they find out that the world is not what they
thought; who find their own way to deal with their harrowing experiences
that doesn't have to be "shoot at/run from everything that has
tentacles!" Thematically, I'd describe it as "Lovecraftian Unknown Armies".
The Seeker's Handbook is pretty barebones in a lot of ways: It consists of the player-facing rules and a long example of play, with no bestiary
and no mythology (for either, you will need the Gardener's Manual). It has
no illustrations beyond the (powerful) cover and a few chapter headings,
and the layout is just text on a page. It's fine, really - a lot better
than thinking you have to throw at least one color illustration on
every page, regardless of the quality of the art.
The core mechanism is: Roll a skill's pool of dice (usually 3-5 for starting characters) and count successes (all dice showing a 5 or 6). One success is usually enough to, well, succeed. There's some twists (if you roll with disadvantage, only the 6s count, if you roll with potency, double the number of your successes after the roll). Beyond the skill list, there's three core attributes (Physicality, Acuity, Willpower) - usually, you don't roll these, but they serve as Hit Points for your body, mind and soul, and you can spend a point from them to buy a success in a skill roll. They're also tied to a neat little mechanism called Challenge, where you roll a test of them to avoid suffering a narratively defined consequence - if you roll too low, you can decide to either suffer the consequence or lose points from the relevant ability. This is mostly used for interpersonal skills - if you're trying to talk someone down, for example, you'd roll your Persuasion or Charm skill, and your successes set a difficulty to their challenge. Now they roll to see whether they shrug it off or whether they have to decide between lowering their weapons or losing a few points of willpower (which might lead to them being penalized on further rolls).
(This mechanism, by the way, reminds me of the Maneuver rules from the Gumshoe RPG Swords of the Serpentine; There's a very neat bidding element here, that can accomodate a lot of narratively different stuff with just one type of roll.)
Combat is an extension of the core rules and keeps it simple and sensible: You won't be dodging bullets here, though you can run for cover. On paper, it looks pretty deadly - or at least, scarring. If any of your 3 attributes falls below zero (-5 means your dead), their negative value is converted into afflictions at the end of the scene, which linger. If your've been shot and are at -3 Physicality, this will haunt you at the very least until you've spent three Downtimes recovering - with a Downtime being a flexible amount of time, but it is assumed that it is about a month of not doing anything terribly stressful. So chasing someone, you might end up having to give up because two months ago, someone shot you and you're still not quite over it.
Mind/Soul damage works the same way - you don't have a sanity score, you just get scarred mentally; most of these scars just take a long time to heal, but they can be permanent as well.
There's a hint of a "class system" in M&P as well, though it is only relevant for advancement: You choose a "study" (stuff like Marital Arts, Scholarship or even getting wealthier) which is your current focus. Whenever you engage with a problem in a way relevant to your study, you earn XP which you can spend on advancement rolls and some special abilities provided by your study. (alternatively, you can spend advancement rolls to recover attribute damage if there's no time for Downtime). Advancement comes with some hard choices, especially because even paying XP, you still have to roll whether you succesfully advance a skill, as in BRP games. Maybe it's a little too harsh, I don't know ... that's something I'd have to find out in play. It is, however, very thematic and flexible (you can change your study at any time) while still giving you some kind of framework for advancement.
Regarding gameplay: When it comes to investigation, M&P takes a different route than the Gumshoe engines, where you get all essential clues for free. M&P is a lot more old school in that you usually have to roll to get information and might very well fail; but it also assumes a more sandbox scenario structure, where there's always another clue to chase down, where the characters lead the narrative and try to make whatever skills they bring to the table matter. I'm not sure it's for everyone, but the good thing is that as with practically any system, you can always apply the "Gumshoe rule" to M&P and just give the players what you consider core clues.
All in all, the system is well designed and very evocative of the kind of fiction it seeks to emulate; It's no terribly detailed, and there's certainly a lot of edge cases necessitating GM fiat, but the system looks robust enough to support all kinds of on-the-fly solutions. Oh, there's also an abstract wealth/income system included that, on paper, looks like it might actually provide results that make sense (which would be a first for me).
The long example in the end does double-duty as a kind of opening fiction: It's both atmospheric and makes sense as a gaming session, and it gives a good idea of the kind of horror that M&P emulates (hint: no tentacles to be seen).
I haven't read the Gardener's Manual yet, which contains all the magic, the creatures and other mythos, supernatural and GM only stuff; to get a full-fledged horror RPG, you obviously need both. But even on it's own, the Seeker's Handbook provides a concise and original system for modern-day campaigns that knows what it's doing, and if you're just playing a gritty modern-day campaign or feel fine with coming up with your own horror mythology, it might be all that you need (though in the latter case, you'd have to design your own magic, creatures and advanced studies, so you'd probably want to get the Gardener's Handbook, anyway).
Here's the authors landing page on drivethrurpg, where you'll find the Seeker's Handbook, The Gardener's Manual and the introductory scenario The Thing That Comes in Autumn.