TL;DR - goodreads.com
In a bit more detail...
I've been using amazon's recommendation engine to find new authors to read, but they changed it, now it sucks. You used to be able to look up all the books you read, rate them, and then get recommendations based on your ratings. Now you can only rate books that you purchased at amazon. >.< There's something called amazon betterizer (horrible name btw) but it just presents 6 books at a time chosen from god knows where and gives you a chance to 'like' them. So you keep getting spammed with best sellers from genre's you don't even read, and there's no 'hate' button to get rid of Twilight and books like Twilight, so the recommendations are terrible now.
I decided I needed a better recommendation site, and I'm amazed at how long it took to find a good one. My first try was Shelfari, which is actually associated with amazon. Pretty standard interface, search for a title or author, click to add it to your shelf, with an option to mark it as one of your favorites. I added a hundred or so, and asked for recommendations. There were none. Ah well, maybe they only do it every few days. Checked back a week later, still no recommendations. Utter disappointment. Then I started to figure it out, this site doesn't pay attention to what books you read or add to your library or rate. You do this to let your friends know what you read! And you invite friends to put up their libraries, too. Then the site just shows what books your friends are reading that you haven't yet. Terrible. >.< I want netflix style personalized recommendations.
The next site I tried was What Should I Read Next. This site has a really crap interface, I have to say. It reminds me of a test project I did back in high school when I was learning how to do sql. But at least it gives you recommendations. You search for a book, click on it, and it gives you other books that you might like. You can add them all to a big list and find books that are... I'm not sure. I check a bunch of books I like, tell it to find me more. Some of the results I recognize. Some of them I have no idea why they show up. Cookbooks and manga. I suppose somebody who likes one of my books must like this manga series. I was able to mark a few as books I didn't want to read. So how do I search for stuff based on the ones I like and the ones I don't like? If I check the rows of books I marked as don't like, will it find books recommended for liking those, or exclude books that are similar to them, or what? This site fails because I simply can't figure out what's going on. Do I want to build a big 'dislike' list, or will that skew my results? Not to mention that the results list is exactly what it sounds like, just a list of titles and authors, no indication what genre or even if it's an actual book, let alone a nice synopsis and cover art. Fuck it, I'm going somewhere else. A higher quality site.
In a search for a site that doesn't look so terribad, I stumbled over Library Thing. But then I realized it was no good. First of all, it boasts that it can connect you to people who read what you do. I'm not looking for a social networking site, or a book club, or whatever. I'm looking for good books to read. Reading a bit more, I found out that it's a pay site. >.< For a service that's clearly being offered for free on several other sites. I left this one quickly, not wanting to pay for something that would turn out to be another friend connect site like shelfari.
Was getting very frustrated at how difficult it was to discover something that seemed so simple, and cursing at the whole social networking thing invading my book recommendations, when I found GoodReads. This site does it right. First of all, it asks you to rate some books and then displays a list of genre's. No more cookbooks, I can just go straight to the section I'm interested in. Then it shows a big page full of book covers, with mouse-over synopsis and details, and under each cover a 1-5 star rating. Finally! No more of these stupid 'likes', I can actually rate things. What I found most impressive was how when I rated something highly, a 4-5, it would insert a new row one down into the grid of books, 'people who liked <title> often like:' these other books. What impressed the hell out of me was that after clicking the first book I found that I'd read, and following the chain of new rows, by the 5th 'You might also like' it had already found two of my favorite books! XD
Continuing with goodreads, I rated a couple hundred books, marked a few as 'I intend to read this but can't rate it now', and asked for personalized recommendations. The recommendations while I was rating were really nifty, but they were global. Of everyone who likes this book, those people also tend to like these other books. Now I wanted something more personalized. Given my ratings of these 200 books, they predict how I'll rate these other books, and then give the top 50. Ran into my first roadblock with the site, it was only giving me recommendations based off my 'I want to read this' row, but the recommendation page told me what to do, saying I needed to select my favorite genre's and then it would recommend based on those genre's. Don't think that's all very necessary, but whatever! Did that, went to the recommendations page again. Got a message that it was building recommendations for me, and will be done in two minutes. Ok. Came back a bit later, and it was exactly what I wanted! Recommended books, with a couple that I'd already read but forgot to rate in the top 10.
So now I've got a great new recommendations site, and a list to take over to half.com. XD