I have book of the Manor .pdf - should I get Book of the Estate as well?

I hope this is the right spot to ask this!

You should, to support the game. :P

The (newest version of the) Book of the Estate is very nice, detailing how the knightly economy now works and having lists of household members. It also has the new improvements and fortifications that have been sanity-tested.

However, it is a supplement. You don't REALLY need it; you can run the game without it. That being said, I strongly encourage you to search this forum for threads where people talk about these two books. In particular, Book of the Manor's variance of Harvest income is WAY too much. Here is a thread I started discussing this very problem and offering some house rules to fix it:
http://nocturnalmediaforum.com/iecarus/forum/showthread.php?2234-Harvests-Raids-Pestilence-and-Wasteland

We have been using the Harvest system described in that thread for almost three years now, and I have been very happy with it.

So, in short: if you have the money and the interest, by all means, go ahead and buy Book of the Estate. It is now better than Book of the Manor in my humble opinion, and I would definitely recommend it over BotM. At $25 it is probably less than pizzas for the game night, and will give you more lasting enjoyment, too.

Thank you that is very helpful - I am looking forward to really getting into this game!

Some added thoughts:
The book of the Estate and the book of the Manor have a different approach to the finances of the holdings of a knight. I have not used either at this moment, but from a readthrough I think the BotE is the more elegant approach, so I would go for that one as well.
BotM: Single manor house and an income dependent on weather and other conditions. It does not take fully into account the size of the household and the costs involved.
BotE: This is an approach where the income is based on lots. Also it is possible to have other income than the standard land, like shipping rights or such things. Also the household is much more defined and have a cost to it.

(I use a modified system of the old Lordly Domains book (I would not look at that as an option though), but are currently considering changing to the BotE system)