Solipstry is a crunchy and flexible unisystem for tabletop rpgs. It shares a lot of design dna with 5e, but its focus is on giving you everything you need for your game to feel gamey, and then getting out of your way and letting you tell your story.
The PDF is 100 pages, with a high density of excellent art and a very solid layout. Every page looks good, and the illustrations are gorgeous.
The rules are semi-complex. If you've already read at least one trpg, they shouldn't be tough to follow, but if this is your first you might have to take a few minutes to puzzle out the book.
If you've played DnD, most of this book will feel like a very familiar landscape. Things here are calculated differently, but there's still AC, fort/ref/will, Hit Points, Initiative, etc.
That said, there's a lot more character customization in Solipstry than in unmodded 5e. Talents are 1/level, and there are also smaller bonuses that you get per point of skill or stat modifier. Races are created from the ground up, not selected from a pre-existing pool. Equipment can be heavily tailored. And there's a boatloat of Talents and Abilities (Solipstry's spells) to choose from.
Characters have a Luck stat, which can be used to affect rolls, and which also determines things like encounter tables and drop rates. Affecting rolls is clear and mechanical, but beyond that most of how Luck functions is up to the GM. There's a lot of that approach in Solipstry, and it expects individual groups and GMs to all put their own spin on the game.
Character progression is very granular, and relies on passing skill rolls. Each successful roll adds a tally, and if you get tallies equal to your skill modifier, you raise the skill. Raise a skill ten times, and you level up.
Combat is grid-based and uses move/minor/standard, following 5e pretty closely---but with a lot of little modifications. Your Luck stat gives you Fortune Points, which you can use to take additional actions or improve your rolls, and your Enlightenment gives you a pool of points you can spend on a la carte bonuses called Truths. Enlightenment recharges easily, and this gives combat a sense of flow and gives players a good feeling of control over their characters and their resources.
For GMs, a lot of advice is provided throughout and a small sample bestiary is included, but this is really a game that wants you to build your own custom elements. There are a lot of sample settings, but they're all short, intended to be used as jumping off points and not complete references.
There's a huge rules and terminology reference at the end of the book, and there's solid character sheets with a lot of tightly organized space to write in, so Solipstry gives you all the tools you need, but it's also looking for GMs and players who enjoy the act of building and balancing game elements.
Overall, I think Solipstry is a solid engine. It opens up elements of 5e's system to make things more hackable and setting neutral, but it still keeps the overall structure. It also introduces elements like Truths and Fortune to make the gameplay less dependent on rolling a good number, and more centered on making good choices. If you're looking for a crunchy grid combat game for your own custom setting and characters, I'd recommend giving this a look.