I'm worried that doing this will come across as spammy/sales-y. 

I definitely appreciate the enthusiasm in this post, I'm excited about Will's book too. 

However, for the reasons Linch shared in their comment, I would recommend editing this post a little.

 I think it is important to only recommend the book to people who we know well enough to judge that they probably would get a lot of a book like this one and to whom we can legibly articulate why we think they'd get a lot out of the book. 

A recommended edit to this post

I recommend editing the friends bit to something like this (in your own words of course, my words are always lacking succinctness so I recommend cutting as you see fit):

If you feel comfortable reaching out to a couple of friends who you think might find "what we owe the future" a good read, now might be a particularly excellent time to give them this book recommendation. 

Now seems like a particularly good time to give this book recommendation to friends who might be interested because the book is getting more media attention than it likely will get in the future because it just launched so there is a lot more media coverage on it than usual. Therefore,  they are more likely to be reminded of your recommendation and so are more likely to actually read the book if you recommend it now than if you recommend it later. 

Having said that, after you have read the book or after you've come across something they'd find  interesting might possibly be better times to share it with certain people so it's probably best to make a judgement call based on the particular friend you are considering recommending the book to.

Other thoughts on who to recommend this book to and how to do it in a sensitive way that leaves room for them to say no if they don't feel it's their vibe

I think it's particularly good to recommend the book to people to whom you can explain clearly why you think they would get a lot out of the book. I also think it could be very good to explicitly encourage them to think critically about it and send you their critical thoughts or discuss their critical thoughts with you in person. If you send it to people you know well enough to have enough context to tell they would probably enjoy the book, you're almost definitely going to genuinely want to hear what they have to say after reading it so that's a double bonus that you've started a conversation! 

I think requests like this can come off as pushy if the person who you are recommending the book to doesn't understand why you think they should read the book. By making the reasoning clear and also leaving room in the message for them to just not follow your recommendation (by not assuming they'll necessarily read it just because you think they'd enjoy it), a recommendation can give a good vibe instead of a bad one. Basically, it's important to vibe it and only recommend the book in ways that leaves everyone feeling good about the interaction whether or not they read the book. 

For example, you could say something like (obviously find your own words, this is definitely a message written with my vibe so it would probably be weird to copy it exactly word-for-word because it probably wouldn't sound like you and therefore wouldn't sound as genuine):

I am so excited about this book being released because I think that future generations matter a tonne and Will MacAskill thinks a little differently to other people who have thought hard about this so I'm looking forward to seeing what he has to say. I thought I'd send you a link too because from previous conversations we've had, I get the sense that you'd enjoy a book detailing someone's thinking on what we can do to benefit future generations. 

I'd be really keen to discuss it with you if you do end up reading it and especially keen to here any pushback you had because I'm a little in my bubble so I won't necessarily be able to see my water as well as you could.

Let me know what you think about my suggestions (and feel free to pushback on anything I've said here). I think I could easily change my mind on the above, I'm not at all confident in my recommendations so I'd be keen to here what you or anyone else thinks.