Systems Skill Books are a market segment that's been surprisingly good for me in spite of constant competition from an obsessive (probably botting) camper and low profit margins per book. I'm not worried about supplying this information because I'm certain if doing so winds up creating so much competition it permanently ruins the market I can always find something else to replace it.
These skills are required to fit subsystems on T3 cruisers. There are 5 systems books for each race (one for each subsystem) and 4 races for a total of 20 different systems books. Schools sell the books for 4.5 million each which places a soft cap on just how much traders can sell them for. Many buyers will pay a little more (even a fair bit more) for the convenience of buying at a trade hub and not having to travel to a school system but most won't pay too much more.
There seems to be one major camper with alts covering systems books with large stacks for sale in all the major hubs and several smaller hubs too. Prices, cut amounts and timing vary from time to time but they match up so well across all the hubs it's highly unlikely different players operating in different places are responsible. He'll usually cut by just 0.01 ISK and do it quickly (with seconds or minutes) but sometimes makes large cuts and/or slows down the frequency of his cuts. When that happens it happens everywhere at the same time.
In my experience (and this holds true in 2 different games), for most markets (there are of course exceptions) campers start dropping out when profits are under 20%. Most campers will disappear once profits are under 10% and under 5% only a few will be left. The less volume there is in a particular market the faster campers drop out and the fewer will stick around to the end.
He likes to buy lowball but he'll generally overcut any buy order under 4 million quickly. He'll go as high as or even a little over 4.3 mil sometimes but slows down over 4 mil and usually stops somewhere between 4.2 and 4.3 mil.
The camper cuts quickly when buy prices are low and/or sell prices are high. When prices get around 4 mil buy and 4.7 mil sell the frequency of his cuts slows down greatly. He will go higher and lower than those prices just not nearly as quickly or as often as usual.
I usually buy for around 4 - 4.3 million and sell for 4.5 - 4.8 million because those are the price ranges where the camper backs off enough I get good volume. My average profit per book is around 500k (10%+) profit though profit per book can be as low as 200k (4.5%) profit. Sometimes much higher, if I can sell for 6 mil each that's over 1.5 million (25%) profit each. The same if I can buy for 3 mil or so and sell near the school price of 4.5 mil.
I started working these books by placing buy orders for 2-4 of each on Jita only. Buy prices were very low, mostly well under 1 million each. A little later it was 4-6 of each on all 5 major hubs with buy prices mostly over 3 million. Then it was 10 of each on all 5 hubs and Lonetrek region with buy prices around 4 mil. Now I buy 20 of each on all 5 hubs plus Lonetrek with buy prices mostly over 4 mil. Prices seem to be pretty stable now, they don't spike all that often anymore and when they do spike it doesn't last long.
Most days my purchase orders bring in 10-20 of each book between all the hubs. I don't have much trouble selling them all within a few days either so I never wind up with too much stock of any one book.
10+ daily volume per book
Those numbers might not look very good at first glance but once you do the math it really isn't bad. Between all my traders I buy and sell over 200 systems books a day. 200 a day at 500k profit each is 100 million a day or 3 billion a month. That's more than enough to cover my monthly subscription costs for 5 accounts and that's a conservative estimate, it's usually higher and can go much higher. With more frequent price updates sales volume, market share and total profits all go up.
The real trick in this market (and most markets for that matter) is setting prices so I get good volume and reasonable profits in spite of campers trying to control the market. If there's too much of a spread between buy and sell prices the camper is all over it and I won't be getting much business unless I camp too. I don't like camping so I squeeze him from both ends (higher buy prices and lower sell prices than he really likes working with) until he backs off, then I make a decent profit.