Ok, for TJ and anyone else building something similar, here are some more photos and notes.
The shelves are indeed 18mm oak faced blockboard. This has an inner core of strips of softwood, running along its length, with two layers of veneer (one cross-grain, one long grain) on each outer surface. It used to be readily available for this sort of project but as discussed
here has virtually been displaced by mdf. It's lighter and stiffer than mdf but not as stiff as solid oak. The shelves are all 1180 mm (46½" ) long and 240mm (9½") deep. (The width was as much as could fit in the alcove and still allow access to a power socket. The depth was just enough for large format books.)
At the front edge of each shelf there's a strip of solid oak, about 19mm square. This was glued on (PVA, no pins) planed flush to the shelves and then worked with a scratchstock to make the decorative edge. The shelves are completely full, with paperbacks on the upper shelves and heavier hardbacks on the lower ones.
The shelves do sag a bit. I think it's just the right side of what is acceptable - but I don't really want to start over again on this piece from over 20 years ago!
This is an attempt to line the camera up with the lowest shelf
and this is the shelf above it
In this shot, with some of the books removed, you can see that I did what I could to stiffen these two lower shelves, by glueing and screwing a couple of oak battens underneath at the back.
The battens are quite small, because that's the wood I had to hand at the time, and because anything bigger would have reduced the space for the larger books, which go right to the back of the case.
I could also have put some intermediate supports onto the back, but it's only lightweight T&G pine cladding and I thought it might bow out a bit or spring out of its bottom rebate if there was too much vertical load on it, so I never did that.
The upper shelves don't have battens but don't really need them with their lighter loads.
I think if I was starting on something this size now, I'd just spend more and get solid oak for the shelves. At the time, I was trying to avoid spending cash - indeed, the whole piece was made possible when I rescued a stack of oak skirting boards from a skip.
While the books are out of the way, here are a couple more pictures of the sawtooth rack arrangement.
The support system is plenty strong enough, even though it's only about 9mm (3/8") thick. I think it's important to decide where you want the shelves to go and make sure that you plan for the final positions when you decide the size of the steps. Once it's all finished and the shelves are in, nothing will ever get adjusted again. Having the shelves demountable is more for the convenience of making the whole piece without wrestling all those ends into place at once to assemble it, and to make it more manageable to pick up.