iUniverse is owned by Barnes & Noble, so you're guaranteed to be on the B&N site. You get Amazon as well as part of the package. My book hit Amazon within a day or two of my receiving my first copy. It's still there three years later. I check periodically. It hit B&N a few days later. I have no idea why they were slower in getting to their own bookseller, but they were. It has continued to be offered there as well.
I've had no problem with the online distribution options iUniverse offers, including the fact that my book is available on the iUniverse site in its entirety for reading by the general public. I've been told by several people that the print was so small, they opted to just order the book. That works for me.
Here's something new I learned, and everyone considering iUniverse should be aware of this: You sign a three-year contract. I was under the impression that at the end of the three years, the contract automatically expires and your book is no longer available. As my book was published three years ago this November, I contacted iUniverse for clarification and to make sure I had the details of the process right. As it turns out, I didn't. I haven't checked my original contract yet, and I don't remember this being in it, but they told me that unless I write in and cancel the contract (30 days' notice required) or they write to me and cancel (same notice required), the book will be available indefinitely.
Now to some people that may not be a positive. For me, it is. It means I can just leave it alone, not have to change the info on my business card or website, and continue to sell my book. I was concerned that I would need to buy 100 copies before it went out of print so I would have a few for the folks who wanted them. I only sell in dribs and drabs--just sold one on Tuesday, in fact, but before then the last sale was through Amazon in July--not in big chunks, and I really didn't want to invest any more in the process while I debate what to do with that book and try to find out what the mainstream publisher who has my second book is planning. I feel as if I got a bit of a reprieve. POD was a mistake, but I intend to make the most of it that I can.
I still will not go POD again unless something in the status of that printing method changes or I find a dead-sure marketing scheme. A niche non-fiction book is difficult to sell under the best of circumstances. At least a mainstream publisher would (hopefully) have a plan for gettng it into the stores where it's most likely to be seen by the people most likely to buy it. This focus on availability at Amazon and B&N is a little skewed. Unless someone is looking specifically for your book, a search for, say, "The Psychology of Sock Colors" may turn up hundreds of titles, and yours may be near the end of the list. Unless it's a small niche or your book is really special in some way, no one will find it. I couldn't find my own book until I searched the actual title
with my name, all in quotes. How many buyers do you think will know to do that with your book? If you're counting on hitting it big on Amazon, you're going to be disappointed. You need a platform from which to hawk your book. The end.
Would I go with Lulu and self-pub? No. There's not enough difference between self-pubbing and POD. Distribution and Marketing are two completely different, though interrelated, parts of the sales puzzle. You're going to do your own marketing no matter which route you choose. Acousticgroupie is right. At least with iUniverse your book can be ordered through B&N stores, but don't count on people not getting frustrated with the waiting period. Only one of my friends tried that, and it took so long for the book to arrive that she canceled the order and I sold her a book through my website. You're actually better off financially buying copies of your book and selling them directly than you are if the buyer goes through Amazon or B&N. The royalty is far lower. You can resell your book yourself at a significant discount and still make more than you will if the buyer goes through any site other than iUnverse.
Just a word to the wise. If your book is good enough to be in print, you really should think about sending out more queries and finding it a mainstream home. You put in the work. It would be a shame to see it languish, and POD or self-pubbing barely count as publishing credits.