InDesign is remarkable. The only time I ever really used it, I was laying out some photobooks (one with photos from our wedding, one with tons of photos of our kid). InDesign has a notion of master templates into which you can just drop text and/or images. The workflow reminded me a lot of the paper-based workflow ("spreads") that we used on my high school yearbook. I imagine that those sorts of people all use InDesign now.

InDesign also has most of the functionality of Illustrator floating around, for when you want to get fancy. In my case, I was just trying to do basic image layout with a handful of captions, and for that it's notably easier than Illustrator to do things like drag and scale an image inside of it's clipping container. There are tons of YouTube videos out there that show you how to drive it and which got me up to speed in a hurry.

Depending on your needs, you may be able to get along just fine with Microsoft Word, Apple Pages, or even PowerPoint / Keynote, which can be tweaked to use paper-sized aspect ratios rather than screen-sized. Those photobooks that I did with InDesign could have just as easily been done with PowerPoint, which was how I started, but PowerPoint chokes when you load in hundreds of full-resolution images, whereas InDesign is comfortable linking to images from external files. Likewise, InDesign gives you a ton of control over the PDFs it writes out (target print resolution, color profile, etc.). I'd imagine that you don't need this level of control for your mailers, but it's really up to you.

Footnote: Sadly, the "pro quality" book printing service I had found and enjoyed has gone out of business. Most of the consumer-grade services want you to upload individual JPEGs and use their templates rather than just letting you give them a giant multi-page PDF. My dearly departed service had simple InDesign templates for all their supported book sizes, marked with where the cutter would go, letting you do full-bleed printing and other goodness. I'm sure there are plenty of other firms out there who do what I want, but I haven't needed to print a photobook for a while.