Guide for Casting
Casting's another great way to make money, and allows you to make those much sought-after items, cannons. Casting, along with Sewing, both benefit greatly from you investing in your home port. At certain levels, investment allows you to buy extra books, and is a very good idea. The job of choice for casting is Mineral Trader, which has Casting, Mineral Trading and Wares Trading as favored skills (if you're working on casting, you should definitely learn mineral and wares trading). Also, keep in mind that the Shipwright's Hammer can give you a +1 boost to your casting rank, which can come quite in handy at times.
For ranks 1-3, I'd recommend making yourself plenty of the products from the Shipwright Basic - Casting book (obtainable at many places, Antwerp's the closest to London). R1 will make carpentry tools, R2 spare rudders, and R3 bells of withdrawal. Lumber for the first two can be obtained up north, Bergen and Oslo both sell it in large quantities, and bronze for the bells can be purchased in London or Plymouth. Also at R3, you can begin to use Firearms Techniques, obtainable from Barcelona and Ceuta. Coal can be purchased at Dublin or Hamburg, and Iron can be purchased in Amsterdam (or London if you've invested a bit and are English, Dublin if you've invested a bit). Casting doesn't really begin, however, until you get it to R5. Once you hit R5, head to Lubeck and pick up the Metal Smelting Book. This book will allow you to make use of the holy city of casting (For northerners at least); Hamburg. R5 and R6 can be quite rapidly (and profitably) passed up by converting iron ore into iron, copper ore into copper, and lead ore into lead. Keep in mind that, even though it's a long trip, Lubeck's a good place to go to from Hamburg, as Lubeck sells iron ore and lead ore (it also sells zinc and copper ore for later). Also at R5, you can make cannonballs using the Firearms book. Once you hit R7, you can begin using the Alloy Smelting book, obtainable by investing 300k in Dublin. This will allow you to make bronze, which is a frequent ingredient in cannons. Both Hamburg and Plymouth sell the ingredients for bronze, but keep in mind that Plymouth also sells bronze, so you'll have to take it somewhere else to sell at profit. Also, at R7, you can begin making arquebuses with the firearms book, which will make you quite a bit of money. Also also at R7, if you can get your hands on a Weapons Manufacturing Techniques book (requires 500k investment at Barcelona, a Spanish territory, or Gronigen, a Dutch territory), a good thing to make here is Longbows, which take one lumber and one iron. At R8, if you're interested in doing this quickly, you can begin making brass using the zinc from Lubeck. This isn't necessary though, as Lubeck is a long ways away. At R9, you can make muskets, a very nice thing to make. They require steel as well as coal and iron, but they're a local good, so you can get quite a bit of free trade fame by selling them for 5k worth of profit at a time. Hamburg and Bremen's markets are quite frequently fried from people over-selling these there, so I recommend going to London or Lubeck to sell.
For Spanish casters, the ideal path's quite a bit different due to the aforementioned Weapons Manufacturing Techniques book and access to the full range of materials from Barcelona. Most of the recipes in this book require steel, which is relatively difficult to obtain for anyone besides the Spanish (the English are #2 at this due to ready access to graphite, but the graphite still requires combination with iron ore to make steel). Barcelona sells steel outright, so Spanish casters may find their time better served by bouncing between Barcelona and Algiers and making whatever the highest recipe from the Weapons book they can use. at R11, Barcelona really starts to shine for Spanish casters, as the Spanish can buy brass there, enabling them to make European Armor, one of the best recipes in the later casting ranks.
Once you get to R13 (or R12 with a hammer) casting, you can begin gold dust grinding. In Havana there's a fixed recipe at the Craftsman that will turn gold dust into gold. You also need handicrafts R5 to use this recipe, and need to invest a bit in Havana before it shows up. While it's nice inherently to have access to the large sums of comparably cheap gold this recipe gives you, the best thing about it is that it'll rank your casting insanely quickly. If you have R5 storage and the Moguls Memoir's storage book (available from Veracruz for 1m), you can use the gold you make from this to make Tribute Goods for Brigands, which sell to players quite nicely.