I've been a freelancer for game companies and worked in book publishing, but I'm not really an indie game publisher. Here are some bits and bobs that might be useful.
DIY. Short of spending a lot hiring people to do everything, do-it-yourself as much as you can. Use as many free and royalty free resources as possible, otherwise you'll eat up a lot of money buying things you don't really need. Start off by actually studying the thing you're going to try to do yourself. See what's being talked about in those areas in relation to your stuff. Making an OSR style game or book? Start by reading up on what's going on with the OSR first and see what the community likes and where their tastes are.
Free fonts.
This is a link to Fontesk's OFL (open font license) fonts. The OFL is effectively the non-revocable OGL for fonts.
You can read about the license here. They are 100% free for commercial use. Though they are mostly display fonts (titles and book covers), a lot of them are also suitable for use as regular book text. There are quite a few lookalike fonts that you will recognize. It's perfectly legal as the original sources these fonts were made from are old enough to be public domain. I'm not a lawyer, but I am a font nerd.
Free images. Don't use "AI" art. It's generated by illegally scanning copyrighted art and generating variations on other artists' work. The legality of this is already being questioned as is the legitimacy of copyrighting "AI" art. It's a mess. Stay away. Lots of museums, universities, and other places offer royalty free art and images.
Here's a search of the Smithsonian.
Smithsonian Open Access.
Here's a link to MoMa.
Here's Europeana.
Here's the British Museum on Flickr.
Flickr Commons. There's also places like Pixabay.
Article collecting image sites.
Layout & design software. You'll want something like Adobe InDesign, but it's expensive.
Here are some alternatives. It's really easy to mess up. Even the big dogs do this sometimes. Covering text with art, picking backgrounds that make the text unreadable, design that's painful to look at, etc.
Editing. It doesn't matter how many times you look for typos and poor construction and transposed words, etc you will inevitably find more after you hit publish. The mind has a fantastic ability to fill in the blanks and make you think you put words on the page when they only exist in your head. You know what you mean or what you meant, so your brain fills in that information...leaving the reader no idea what you're talking about because you didn't actually get your thoughts fully onto the page. Other than going word-by-word backwards through your text to make sure everything is there (yes, that is an actual editing trick), the best advice I can offer is hire an editor. Pay special attention to charts matching text.
Hire help. If you can't do it yourself, hire someone to do it. Even if you can do it yourself, you should hire a second pair of eyes to check your work. A lot of the obvious services you'd need (art, editing, layout & design, etc) can be expensive, but you can also hire students to do this stuff. You don't need pros with 20+ years of experience and you likely couldn't afford them, so look for colleges and universities with graphic design programs and book publishing programs. Contact the heads of the programs and see if you can drum up some cheap help. Be polite and professional and offer actual money. Exposure won't pay your rent so don't expect it to pay someone else's rent. Offering royalties is a no go, so don't. They do the work, you pay them. That's that.
