Yes you can use many tricks, and they work. But in the end of the day you are not different that a czsino, a drug dealer or a tabloid magazin when you use them.
It is more economically efficient to trick the user into staying in the short run. But in the long run, we are using google and stack overflow because they are the most useful, not for tricks.
Among tricks, some useful ones are also often badly implemented and become armful.
Ex: notifications are only useful if it result in more productivity than distraction. I hate most notifications, they disturb my flow, but I like being request them for specific cases to save me time.
A light gamification can be fun, but if it's at the price of my main usage of the site I'll leave.
Basically give me want I want, quickly and in a useful way, and I'll stay. Save me time and energy and i may even pay.
Unfortunately, hn readers are not a good sample for user behavior. We all have ad blockers, we boycott sites with behavior we dislike and we know how to by pass stupid tech decisions.
But if you manage to engage hn users with the same content and design than regular user, your model has a good base to be sustainable because it has credibility.
Credibility is harder to build than addiction. It's not quickly as rewarding, but it more reliable. And more satisfying for you.
I wrote it on a phone to use auto correct and poor keywords as my muses.
+1
Everything else is irrelevant
Figure out how the app can trigger or remind them of these moments. Notifications are the obvious one, but you want to be careful not to train the user to tune out your app if you send too many notifs or notifs they are uninterested in. The best triggers are ones the user has asked for themselves.
For example, when they get a new book you can prompt them to set a goal to finish reading it in a week or two, or whatever is comfortable, and then ask them if they want you to check up on them (notification) when that time is up. If they set the goal, they will probably welcome it.
Additionally, you want to try and build habits that help the user improve his or her life. So if you build a habit of recording their daily reading log, and reward the user for reading every day without missing one, then they are happy because they are reading more books and they are also connecting their new reading habit with the action of opening and using your app.
I would say look at what Facebook does, they are like digital crack.
Your site should be very useful to users and you should try to gamify things a little bit as others have said.
I would recommend building out a segmented newsletter for people. Say for instance, you get a bunch of HN users that signup. If they are interested in startups, you have a weekly or monthly email that goes out that lists books on startups, customer development, lean startups etc.
You give a small summary on each book in the newsletter and the link brings them back to the site.
I have also seen email notifications for responses to forum posts that have worked really well.
One other thing, I tried your site on a Nexus 5, and the images of the books are a little shifted and oversized. I would suggest tweaking this a little as the majority of people browse the web on their phones these days.
For example, if I use the app for X minutes in a day, the next day I will get a summary as a notification.
It also adds light questions, to give an example it takes a random word and asks something like "how do you pronounce this?" or "what does X mean?". Tapping it takes you to the answer. They are made to require as little actual app interaction as possible, kind of making the learning (or rather, retention) almost passive to a degree.
I found the success of this minor so far, but I think it can work fairly well. It just needs a good balance.
* Trigger
* Actions
* Rewards (variable rewards)
Ref book on Amazon: http://amzn.to/2ngLIuz