Note taking is not really stressed by most brewing resources, if it is mentioned at all, but, in my opinion, it is one of the most important elements of the brewing process and can make the difference between producing good beers and producing great beers.
Brewing is a craft and there is a lot to learn. Inevitably you will have your successes and your failures. Your brew book will give you the ability to learn from both.
It is in the nature of the process, that the results are several weeks, or even several months removed from the original brew day, so by the time you taste that fine Pilsner or beautiful Golden Triple, you may not even remember the basic recipe you used, let alone what temperature the mash was at. Without notes, how can you possibly hope to make this beer again?
If you crack open the first example of an ale which has been maturing for a few weeks and it just isn't right, you will find that the first thing you reach for is your brew book. Where else will you find the reason for that off flavour? How else will you know what to avoid the next time?
The mechanics of note taking and the format of the notes are up to you, but make sure that you note down everything. Don't be tempted to gloss over the mistakes you made. Remember, this is your brewbook and you don't have to show it to anyone else if you don't want to, so be honest. That mistake you made might be the key to why your beer turns out a little off, or it could be the lucky little quirk that gives your creation that certain something you really like. You won't know for weeks and by then you might not remember the details.
A typical entry for a brew book might look something like this:
22/8/2006
Grist:
3.5Kg Pale malt
500g Crystal malt
50g Chocolate Malt
150g Raw cane sugar (boil)
Strike water: 12 Litres at 75C to try for a Mash temperature of 68C.
Temperature ended up at 66C. Think I got the grain temperature wrong in my strike water calculations.
Mashed for one hour, during which the temperature dropped to 64C. Look into better insulation for mash tun.
Batch sparged until I collected 25 litres of wort.
Boil.
Added 150g of sugar crystals at start of boil.
Hops:
60 Min: Bittering
Target (11% AA) 15g 21.4 IBU
20 Min: Flavouring
East Kent Goldings (4.5% AA) 15g 3.1 IBU
5 Min: Aroma
East Kent Goldings (4.5% AA) 15g 1.5 IBU
Total Bittering 45g 26 IBU
Had boil over before hot break. Keep a better eye on it next time.
Forgot to add Irish moss. Add 1tsp of Irish Moss at 15 minutes next time.
Chilled wort to 19C, took gravity reading and pitched onto yeast cake of Bitter brewed 8/8/2006. Yeast was Danstar Windsor. Aerated wort by shaking the bucket vigorously.
Original Gravity = 1.047.
24/8/2006 Gravity = 1.022
26/8/2006 Gravity = 1.016
28/8/2006 Gravity = 1.014
30/8/2006 Gravity = 1.014
1/9/2006 Gravity = 1.014
Kegged beer and dry hopped with 15g of East Kent Goldings:
Boiled 150g of raw cane sugar for 10 minutes. Cooled and used it to prime beer.
Sanitised muslin bag by boiling. Added 15g of East Kent Goldings to muslin bag, tied neck closed and added to keg before syphoning beer into it.
Calculated Alcohol content: 4.3% Vol.
The last point I would like to make about your brew book is that, as time goes on, it can give you valuable perspective on how far you have come. I recently looked over an old brew book and discovered the notes I made for my first all grain beer. I nearly spat perfectly good beer all over the page when I saw the mountain of grain I used and the pathetic original gravity I ended up with.