You want "the best", or "the best within budget"?
For backs, there's the IQ180, but there's also the new IQ280. The main advantage of the 280 over the 180 is that the 280 doesn't crap out on long exposures, get one of them for 1min+.
Going the Hasselblad route, there's the H4D200MS, it's a 50MP sensor that shifts itself by 1/2 a pixel in between 4 shots and stitches them internally to a 200MP image.
Or there's the Leica S2 and Pentax 645D. Both are 'only' 40MP, but for anything less than A2 size you can just as easily use a 35mm 20MP sensor.
Bodies, there's the Mamiya / Phase One 645DF, or any Mamiya 645 with a D in its name (ie all but the 645AF). If you're tripod-mounted you probably won't need any of the functions that come with the newer bodies.
Or there's the Alpa 12FPS
, a camera designed to have higher IQ by having less shutter-induced shake (less than even a leaf shutter).
Hasselblad, Leica, and Pentax you're stuck with the bodies that you get.
Lenses, the Leica S2 has Leica quality glass. 'nuff said.
Hasselblad, you're getting their name-brand stuff too (used to be made by Zeiss, now by Fuji, either way, it's damn nice).
Phasemiya, there's the 28mm LS which is very sharp but possibly too wide for you, the 'new digital' 35mm I haven't tried but I definitly wasn't impressed by my 'C' copy of the old version.
Alpa, you can use their extremely nice (with pricetags to match) LF 'alpagon' lenses (rebadged Rodenstock Apo Sironar HR). Or you can get it with any mount you like, even EF.
And on all of those bodies you can use lenses with a longer flange via adapter, like Hassy V, Pentacon 6, Bronica, Rollei 6000, Pentax 67, etc etc...
Or realistically, a 6D with a 24mm TSE or 35mm Sigma/Zeiss is going to be more than enough for a simple book that doesn't cost $5 per page to print at stupid resolutions...
ps, a 25-35mm lens on 'FF' (24x36mm) equates to a 38-55mm on '645' (56x37mm once it's cropped, I'll presume you're cropping more panoramic than 3x2).
But don't forget digital 645 sensors aren't as big as 645 film, depending on the sensor you get you may have to get a wider lens to account for a smaller sensor...