I understand that grimoires give me access to skills that a character wouldn't normally have, but I don't really understand how combining them works, or what the "equipment bonus" does. Can an existing grimoire get better on its own, or only from combining it with another? I'd like more info on how these work.
So when you combine grimoire stones, you need at least 3 stones, and it results in 1 stone. The first stone you select decides how many skills the resulting stone can have. I.e. if you pick a stone with 2 skills on it, the resulting stone can have at most 2 skills. Then, the first TWO stones you pick decide what skills the resulting stone can have. For example, your first stone has Provoke and Front Guard, and the second stone has Fortify. The resulting stone can choose from Provoke, Front Guard, and Fortify for its skills. And finally, the third stone is there only to add an equipment bonus; its skills do NOT matter. What this does is allow a character to equip items outside of their usual allotment of gear. Take Simon for example; he comes with a sword stone, which is why he comes with a Scramasax, even though he's a Medic, which can't normally equip swords.
So let's run through a full synthesis here. You've got 3 stones, and they're chosen in this order. First, a stone with two skills, Provoke and Front Guard. Second, a stone with one skill, Fortify. Third, a stone an equipment bonus for spears. With these stones, you can make a stone with 2 skills, choosing from a skill pool of Provoke, Front Guard, and Fortify, and it will have a spear equipment bonus.
Also, no, a grimoire stone cannot grow on its own, only through combining stones.
Hope I answered everything satisfactorily.