One advantage of Tonk strip (he said, getting ready to drop everything and run) is that you can flip sagging shelves over, so that they bend straight again (well, at least for a while). I've done this with our bookcases, but I had to admit defeat with our 1924 Britannica set after a collapse: just too heavy for the shelves' width.
Which made me wonder: Has anyone used L-section aluminium, brass or possibly steel reinforcement at the back to improve rigidity? I've done this successfully with Ikea, 1200mm wide wardrobe shelves, but the angle strip was fixed to the carcase, not fitted into a rebate under the back of the shelf. I guess you could even let one side of the "L"project above the back of the shelf (it wouldn't be seen behind the books), thus improving the rigidity further.
I know it's not neat and elegant as a woodworking solution, but it is strong. We have a lot of "coffee-table" books, and when they first started coming into the house, decades ago, I thought we'd have then for a while and then they'd end up in charity shops, but that's not been the case: some of the go-to reference workd are really quite big, and I've got some Ansel Adams stuff (for example) that I hope to bequeath to the kids. And academic works often aren't paperback-novel-sized, either.
I think the "Penguin" size was so that the book could fit in an overcoat pocket. The reason that bookshops get away with standard shelves is that the majority of their stock-turn from "impulse" purchases comes from paperback novels, and anyway they can't sell from high shelves as easily as the ones that are at, or slightly below eye level, nor can they alter shelf sizes easily. They tend to put the bigger books into an 'oversize' section, or simply pile them on a table - no an option in our lounge!*.
Bristol is rather depressing nowadays - in the 1980s there were five bookshops (at least) on Park Street. Now there are none. Thankfully, our little one at the end of the road is still going - I can get new titles from them faster than from Amazon, and often cheaper, so they are certainly not redundant!
E. (sorry - wasn't trying to steal the thread. Ignore everything except the bit about Aly angle!)
*Most of the tables already have books on them!