I've used a whole bunch of software to make books, and I have to say that there is no true alternative to using InDesign- it is the most widely used in industry for a reason, but that's primarily because it is way more powerful than any of its alternatives and is primarily designed for the professional publishing houses. I also have to disagree with the steep learning curve idea, it is actually one of the simplest pieces of software to use, primarily because the user interface is designed with professional use in mind.
That being said, there are still a lot of good alternatives out there. No photo-editting software is as capable of photoshop, but lots of people don't have or need it.
The most common alternative is obvously the built-in part to iphoto and aperture, which I dislike because you get almost no control over page layouts. Lots of people use Blurb, so BookSmart is an option for them, but I find the interface a little clunky and slow. Pages is another option, I find it a bit slow (especially with a lot of full resolution images), but is overall not too bad. Finally, there is PowerPoint, which is far more capable than most people think. I would say it runs better than most of the other options here, and is at least as powerful than any of the other options barring InDesign. Any of these options will work, but I would say that if you want to do this frequently, you should just get InDesign and save yourself the trouble. I would avoid any web-based tools, as they are much slower than anything else.