Never had any problem writing books in LaTeX. The main difficulty has been in deciding whether I want a modern or medieval structure.
Docbook, on the other hand, I hated. I helped with the writing of a few chapters of the Linux Advanced Traffic Control book, which was abandoned in part because Docbook was such a disgusting system.
XML is useless for typesetting. It's not really that useful for organizing anything - you'll have used XML-driven databases, but you'll have never used an XML-driven database that had any performance or serious functionality. (LaTeX doesn't do databases, either, but it doesn't pretend to. It has external engines for databases, which are actually quite nice.)
Web pages? Never had any problem embedding HTML in LaTeX. In fact, I have very very rarely found ANY document style to be LaTeX-incompatible. Load up the correct document type, load up the appropriate stylesheets and you're good. Yes, spiral text is hard. Yes, embedding HDR images can be a pain. Yes, alpha blending isn't that hot. But how often do you use any of these for owner's manuals or contracts?
There are more table classes than I'd really like, and some of the style coding is scruffy, but I challenge anyone to find a genuine, common document type that LaTeX* cannot do as well as or better than any non-TeX wordprocessor, DTP solution or XML-based system. (Non-TeX means you can't compare TeX with Scientific Word, TeXmacs or any other engine that uses TeX behind the scenes.)
(To make it absolutely clear, "as well as or better than" can refer to any one or more parameters. So if I get better-quality output, that's better than. If I can achieve comparable results with cleaner, easier-to-maintain syntax, that's also better than. To win, your solution has to not merely equal but actually exceed what I can do on EVERY parameter, or you have failed to demonstrate something that supercedes.)
A bitcoin to anyone who can do this.
*I am including all dialects of LaTeX here, so LuaLaTeX, PDFTeX, etc, are all things I can consider on my side, as are all WYSIWYG and WYSIWYM editors, Metapost, supplemental services, style sheets, etc. Since this is versus a specific alternative, anything comparable for that specific alternative is fair game for you to use, but you can't mix in other alternatives. It has to be one versus the complete TeX family if you want to prove your point.