I've been making some use of Google's Play Store book recommendations system. If Amazon's recommendation system is problematic, Google's system is just ridiculous.
First of all, if you mark that you're not interested in something, you want the system to stop recommending it. Google's system seems to wait several days before an update, and then the same books seem to come back at random after a while. I swear, it has recommended "Spock's World" to me ten separate times (and with the exception of John M. Ford books I don't read Star Trek novels).
Recently, it doesn't seem to remember updates at all. They grey out for a few hours, and then come back. If I make an inclusive rather than an exclusive change (e. g. buying a book) new books get added, but none are ever deleted because I say I'm not interested.
It even recommends a book I reviewed with a fairly negative review on Google Books. One part of the system clearly cares nothing for explicit information in another part of the system.
Secondly, even Amazon distinguishes between things you don't want and things you already own. Google just has a single "not interested" option, with no way of indicating whether you aren't interested because you already have it (in which case you might be genuinely interested in similar books) or because you consign all works by that author to the outer darkness (I keep having to individually mark off books by MZB, John Norman, and OSC, and even then there's no distinguishing between those and my marking off Neal Stephenson because I already have all his books). Rating really isn't the answer here, especially when I'm marking off books I haven't read and have no interest in reading.
Also, given their resources, their selection is incomplete. I was looking for a book by Maitland on canon law in mediaeval England and could not find it searching via the play store interface. I did find a copy at archive.org ... which had been digitally converted by Google. (Not a clean copy - their software needs updating to handle italics and, even more, Latin - but usable nevertheless.)
Oh, and if I download one variant of Autenreith's Homeric Dictionary, I really don't want to have seven other versions of the same book (very slightly different editions) suggested to me. On the other hand, not recommending Liddell and Scott seems a little short-sighted.
For a company which built its reputation on sophisticated analytics, their recommendations really disappoint. If this represents the sort of sophisticated algorithms that go into ad placement, we don't have that much to worry about.
On the other hand, some of these things are so obvious that maybe they just don't care. I'm willing to bet on this second possibility. The critical parts of the play store are apps and music for Google: they're key competitive features with a mass audience. Books are there to stake out a position vis-à-vis iTunes more than an attempt to build a serious online bookstore.
I did register a complaint once. I ended up texting back and forth with a representative who wanted me to do their work in detailed testing rather than accepting the bug report as customer input. No, I do not have time, unless you pay me, to do the work your quality assurance department should have done long ago.