Go with red path. Yes it's doable, and yes the lego book is worth more.

i'd definitely go with the blue. i dont think the essences are all that valuable (honestly, with the way the pricing structure works the silver ones are way more useful than the gold), but with a surplus of books like you or i have an extra lego tome is practically worthless.

I did the red path leading to the leggo book so yes, it's possible.

As for your second question - what do you need, aside from good legendaries? If you're not interested in leggo books or ascention, that leaves only chaos ore. I'd go for that, then.

In essence @kramaswamy.kr is correct that a leggo book is worth much more than the essence.  But that doesn't address your needs. 

lid I have a leggo I want to build...

levels, ranks, ascension, masteries

books

blessings, an afterthought.

either path you choose, you are hording for the future.  Your bottleneck is lack of leggos.  I will not advise you on what you should decide, but if your pantry is full of books, and empty of essence?

Well, I'm not seeing the essences being all that useful in the near future since I've only got a few champions that I've even been able to awaken more than once (aside from a lucky 6* for Fenax, who now hits like a truck in DT). They may become relevant once I actually start hitting the twins and can gather more of them, but right now there's a fairly big risk that they'll just be sitting there for a year before I get high enough souls on relevant champions and enough of them in total to actually be able to use them to buy a better soul.

So in that respect, the book has a higher chance of being useful at some point soon. I could get lucky and pull a number of useful legos because I really don't have that many (and I've still managed several dupes somehow), but I doubt I'll fall into a big pile of essence. The only reason I'm even considering the essence is because I seem to be able to get books while the essence I don't, but that probably just means I'm overvaluing it and by the time I can actually use it, I'll have an easier time getting more of it anyway.

I'm leaning towards the book. Essence seems like more of a gamble.

I'll finish up the book today, so I chose the far right path myself. :)