I have been tailoring since I started sewing. I have a stack of trusty books with everything anyone could possibly need to know in them. Some of them can be found in local dead tree repositories (libraries).
While a lot of it translates just as easily to making BJD clothing, not all of it does completely. You can't, for example, put all the layers of hand stitched padding that make the perfect shape in a man's bespoke suit in a doll's suit. It would add a lot of unnecessary bulk. Cheat and use a good knit iron on interfacing instead.
For instance, welt pockets. With the larger dolls, yes. With anything under 60cm, I'm inclined to just use the mock pocket and call it done. I've noticed a lot of the 60 cm boy dolls have huge hands compared to the proportions their pockets would be on their garments. A patch pocket or flap for style, but if it were in proportion with the hand, rather than the garment, it would have to be huge.
It's safest to use only the thinnest tropical weight suiting and very thin lining can be used for dolls. Sometimes that means the fabric frays as fast as you work with it, so you have to take extra time to go around every piece with Fray Check and be extra careful about using good tight stitches.
It takes a lot of patience to properly sew any garment to couture standards and only about 50% more to tailor a suit for a girl or boy doll. It's much easier than tailoring for a human. The trick is assembling garments for dolls follows a slightly different path than for humans. Small sleeves are tricky to turn right side out and getting linings to hang right without bagging the fashion fabric is difficult without a LOT of pins and some unfortunate swearing. (It helps, really.) And most good tailoring books assume you don't need an actual pattern, because if you're reading the book, you know how to measure and draft a suit pattern by simply doing it. There really are only a few main changes in men's suit styles. All the fashion of the past century is just a matter of details and fit.
Suits go from loose to tight, depending on the season and whim of designers, but the basic pattern is pretty much engraved in stone. The Jacket: a front, a back, sleeves and a collar. The Slacks: Back and front. The Vest: back and front. Everything else is just where you put, or don't put pockets, double breasted is just a bit more overlap and western is just a shape of a front pocket or yoke, a tail coat is just a jacket with its front cut short and its back lengthened.
Once you've made a few suits, it's just like any other sewing project. Lots of sewing, lots of pressing and attention to detail in the shaping and line of the fabric will get you--a suit.
The biggest problem with making Doll sized suits is the buttons. It's difficult to find buttons that are a tiny 5-6 mm. I use Barbie size buttons for the cuff trim buttons, but unless the companies making doll clothing have someone making buttons specifically for them, I haven't found anything that works that's an actual button to start with except the plastic ones I'm using now. I'd love to be able to find brass ones. I'm going to try using spacer beads, with another bead in the middle to make the shank. But that's up in the air right now.