🚀 go-pugleaf

Ariel, 

      although the 101 on how to prepare bleeds wasn't necessary




Good. <g> 

      How to communicate your "trim" intentions with the printer?




Give him the specs for the journal: "28 x 20 cm trim size, horizontal, plus bleed." Although, at least in the States, the horizontal dimension is typically provided first. 

      the couple of dozen books I've prepared, I've usually had to prepare a
      special diagram showing the distance from the end of the text line to
      the edge of the page, plus the distance to the top edge, etc., which I
      send to the printer, or plate-maker.




While it certainly can't hurt, with any decent printer, this would be a totally needless exercise. Your InDesign or QuarkXPress layout pages as you've defined them, or press-quality PDF or PDF/X1a files made from them, would clearly show him exactly where the crop/trim is in relationship to the text or any other graphic elements on the page. 

      I have occassionally tried simply to enable the option in InDesign to
      add crop marks when creating the PDF. However, the printers tend to tell
      me that this isn't helpful. Not sure why not?




Ask the printer. The only reason that comes to mind is that when they do page imposition, they have to recreate crop marks to reflect the layout of the entire signature. But I see no harm in doing so, as there then won't be any confusion if any of the pages includes bleeds. 

      I've had some experiences where I took care to design the book with non-symmetrical
      margins. I added crop marks, sent the file off, and got a proof back with
      symmetrical margins. Printers seem to assume that margins are always symmetrical.




Sounds like you're dealing with some seriously challenged printers who don't bother to check your files and your supplied printouts against their proofs. How do you say, "Idiot!" in Hebrew? <g> I've never had to tell a printer what the margins are. He follows the layout in the file, or full-size printout I send along with the files. 

      I don't know if you do much by way of 200+ pp books, but presumably the
      principles are the same whatever the content or length.




None. But, yes, the principle is the same, whether doing a simple flyer, an advertisement, stationery, or package design.

I hope this helps.

Neil