One slight amendment, you are right with everything except for that perfect bound, or PUR bound books aren't folded into signatures, that isn't true.

The sections of the book will still be folded into signatures, as you say 32pp or 16pp sections are the most common (pp being printed pages) they will then be collated down a line, so you have a feeder that feeds each individual section in the order that they should appear, once you get to the end of the feeder section all of them will be held together tightly by a clamp.

This clamp then brings this book block around against a milling blade which will then mill a couple of mm off of the entire spine section of this book block. This then essentially turns them all into individual leaves as they are no longer now joined together at the spine where they are folded. The clamp then runs around and rolls the spine up against a glue applicator wheel. This will add a layer of glue, the thickness of which can be changed by the operator, to the spine of the book block.

The block carries on in the machine to the cover feeder. Here the cover feeder will feed a single cover at a time through two creasing wheels that will crease the cover where it needs to be folded. This again will be adjusted depending on the thickness of the book, straight after that it will add a line of side glue to either side of the scores that will, surprise surprise glue the cover to the sides of the book block. The cover then moves down between fingers that guide the front and back cover in the direction they need to be folded before the book block comes up to meet the cover.

As the two meet there will be a plate that adds some pressure to aid the bonding before the clamp releases. The clamp will return around the machine to collect another book block.

The book is then dropped down onto a long conveyor belt that travels the length of the machine (the one I'm used to was around 50m long) and back to give the glue so time to cure before dropping into a three knife trimmer to trim the head foot and for edge. This will trim the book in singles or multiples at a time depending on the book thickness.

Then they run into a stacker to be stacked ready to be ejected.

Source I working in industrial print and have run PUR binders and folders.

Sorry this was a long reply just to say perfect bound stuff is still folded into sigs before being bound :)

This was a fascinating read, thank you for explaining in such detail.

without the issue of spine cracking that glued, perfect-bound books are prone to

My first edition copy of The Crystal Shard by RA Salvatore cracked a few years ago and in the last move I lost a page somehow.

Stitched is 100% better AND allows me to easily re-cover an old book.

Rofl... I'm a patient person and even I don't have the patience for that

Edit: I take it back, as @schmidtgenetics suggested, I tried it in my current book, that was annoyingly closing itself all the time, at around 10 pages a go, and it does work really really well, and only toook 1 or 2 minutes.

So, thanks for the tip, I'll be doing this going forward!