Awesome value for a book of building here, skipping a full 2 week upgrade. If I did not rush, I would be wasting a book here on a 6 day upgrade, essentially throwing away 8 days of upgrade time. This is why I am upgrading my teslas and xbows to the penultimate level (TH11 level) so that I can efficiently use 9 books to skip the TH12 upgrade time.
Besides these TH12 upgrades, I would recommend other rushers to use books on barracks instead because it is really annoying to farm without it. I would say use them on lvl12 and lvl13 (miner and electro) barracks because they take a significant amount of time. If your barracks are lower leveled and you have a book, I think the most efficient thing to use your book on is the town hall itself. Only use books if you are going to waste one, meaning you have a book in your storage and about to throw away a new one. For example, you have 3 day event which gives one book but already have a book in your storage, and the event is about to finish.
Furthermore, I have a ton of cheap upgrades right after using the book, so I can start an upgrade immediately. This is actually quite significant because I tried pulling the same trick on my main accounts who maxed TH11 when TH11 was the highest level. The problem is, after spending 12mil on an upgrade, I would have 0 gold left to upgrade anything else. I would then need to farm another 12mil which would take another 1-2 days of farming. That would cut my time skip from 2 weeks to 12-13 days. Small difference but it can add up as I use quite a few books before I was a maxed TH12. At least I’m still more efficient than a TH10/11 maxer.
After maxing walls, my elixir serves no other purpose than to train my army and the occasional lab upgrade. This is why I thought I could graduate to use miners now. Reality, miners are kinda iffy and inconsistent. They are absolute trash without the warden at TH12.
Using miners at TH10 isn’t a problem because you can crush maxed TH10 with relative ease using lvl3 miners and your walls cost nothing. At TH11/12 though, you really want the elixir for warden and walls. If you overspend on miners, your wall progress will suffer. You also cannot easily crush TH11/12 using miners without your warden which you should be upgrading to max asap. Hence why I generally do not recommend miners.
I might elaborate more on this topic afterwards as this post is a little long already
Miners seem good in this example, until you realise that my qw+barch can get the same amount. Still, its something different to barch which is a welcome change after 10 months of barching
Still upgrading xbows and teslas. After maxing one tesla, I upgraded an archer tower.