There are a number of open source tools that might be what you need. The right tool will depend on 1.) Length of your work 2.) the content and 3.) whether this is a one-time thing or you want to invest some time into learning a new publishing method.
Note: I'm assuming that you're targeting PDF.
Great tool with a number of caveats. As you noticed, it will crash sometimes, and it becomes difficult to work with large amounts of text. I've never used it for documents longer than 20 pages for that reason. I do a lot of technical writing, and Scribus is not good with that kind of stuff (page references, autonumbering, etc). You generally have to split your book into separate Scribus files per chapter, which is very annoying to work with in my opinion.
For long documents like books, Scribus works best as the final step in your workflow, not the main tool. In fact, the Official Scribus Manaul suggests treating it that way, as a tool for a very specific job (page layouts). You can use a tool like LibreOffice (or just a simple text editor with good Markdown support) to write your text, create a Scribus template, import each chapter, and then use a separate tool to stitch the final PDF together. This means you're waiting until your book is essentially done before you start producing that final PDF. It can be hard to work this way sometimes, since you may need advance copies or drafts, and it's hard to work hard on something and not start seeing results until late.
This is a swiss army knife for document conversion. This could be used to get your HTML quickly into LaTeX, for example, or ODT. It's markdown variant is pretty good and there are a lot of options. You could convert your HTML into markdown and work with that, then import it into Scribus. Pandoc can do PDF output too (using pdflatex), so in theory you could actually just shove your HTML into it and get a PDF out. You can control the output's appearance to an extent using templates, but Pandoc is probably best used for converting between intermediate formats.
TeX and Friends
TeX (the base engine) and the macro languages built on top of it (LaTeX, XeLaTeX, LuaLaTeX, etc) are ubiquitous for a reason: they are powerful. As you've noticed, there is a steep learning curve, and even with some programming experience, the syntax and features of TeX can be mind baffling.
To learn LaTeX, I used this wikibook and LyX (screwing around with it in WYSIWYM mode, and then looking at the LaTeX source). There are lots of book and resources out there including fabulous the TeX SE. Once you spend a little time with it, it will become easier.
Docbook is an XML tool. You use it in conjunction with the Docbook XSL to produce the output. Docbook is obviously geared towards technical publications and has a higher learning curve since it requires some skill with XML (and customization is a beast if you've never used XSL or FO). Pandoc can convert HTML to Docbook, and there are XSL stylesheets out there that can convert HTML to Docbook, but I've never tried this. You'd probably only want to go this route if you write technical stuff and plan on writing more in the future.
An XML tool similar to Docbook, but geared more towards modularity. There are a few transformation options, with the DITA Open Toolkit being FOSS and pretty powerful. Like Docbook, there is a high learning curve, and you'd probably only go this route if you are a technical writer.
So what to use?
- If you want high-quality and can invest the time, use LaTeX
- If you want to get good results and see the layout as you work with it (and can tolerate its quirks), get all your text into a format that can be easily imported, and then use Scribus
- If you are a technical writer, learn Docbook or DITA
- If you want to try different tools, use Pandoc to convert your existing work.