Ingram/Lightning Source is the biggest independent book distributor for traditional publishers to bookstores. Its self-publishing arm, Ingram Spark, distributes to brick-and-mortar bookstores, libraries, and also to POD online sellers like Amazon and Barnes and Noble. It does not sell books at retail itself, so it’s not like Amazon in that sense. It helps you prepare for distribution to sellers and will provide you with printed copies to sell yourself.
Like Amazon, IngramSpark does not charge a fee to upload your book.
Ingram Spark will also ask you to decide on a discount percentage to the booksellers (KDP figures that in for itself since they are a seller also) and whether or not you want to allow returns.
I found Ingram Spark’s print quality to be good overall. Not way better than Amazon, for example, but certainly adequate at the 50# paper quality they used for both black and white and color printing for what I got from them. I was able to save quite a bit on color printing compared to Amazon simply because Ingram offered the option to use the 50# paper for color, as well as a couple of options with the higher quality 70# paper. I still think the 50# paper version was fine for both black and white and color.
Ingram Spark’s biggest downside is that their customer service is basically nonexistent. They rely on a Facebook group, of all things, as a “community” to deal with questions and concerns. Authors help each other, and Ingram Spark does have moderators and admins who also respond to queries and complaints (eventually). I found this lack of direct support (no live chat, no email, no phone?) quite disconcerting, but I am hoping this will improve as the pandemic recedes in 2022.
Ingram Spark will also set up distribution for your ebook, but from what I’ve read, you are better off doing that directly with Amazon KDP or by using Draft2Digital.
PS: You can also submit your book to more than one printing/distribution company. Here’s an article on the advantages of submitting to Amazon KDP and Ingram Spark for the same book.