UPDATE Mar 11: After further testing, I realise I've been inaccurate to say that heavy washes in general produces puddling stains. This seems to happen mainly with certain fountain pen inks and dyes. But for watercolour, the paper can withstand heavy washes and does not leave puddling stains. I've amended this in the review below.
I've been using the Stillman & Birn Gamma series sketchbook for about 2 months now and I must say it IS as good as people say it is. The cover is hardbound and it looks like leather (I don't know if it's real leather or not). It feels nice and sturdy. There are 62 sheets of archival paper inside. Which means you get 124 pages. The paper is a very very nice ivory color. That's one thing that helps make the drawings in this sketchbook look good. Ivory is soothing to the eyes opposed to glaring white sketchbook papers.
Let's look at the specs. As it says, the paper does hold to quite a bit of rubbing with an eraser. And I must agree that it does have a very nice toothy feel when my pen or pencil draws over it. I'd describe the feeling as "crisp."
On the cover, it says that it is "suitable for all dry media and will accept light washes". I've tested it out and found that it's good for both dry media and also good for heavy washes. From other reviews with this sketchbook, there has been success with using heavy washes on this paper.
Firstly, pencils. I don't draw with pencils much. But it was a pleasant experience drawing with pencils in this book. The unique tooth of this paper does catch the particles of dry media well as you can see.
Next, I tried various inks. I tried various fountain pen inks, indian ink, chinese ink, pebeo watercolours and never once did the inks bleed through to the other side. I didn't notice any feathering either. That's really good. I've tried these inks using fountain pens as well as dip pens that scrape the paper and the paper held everything very well.
I noticed that certain fountain pen Inks and inks like Pebeo tend to puddle and leave puddle stains on this paper easily when it comes to heavy washes. I suppose it is because of the shading properties of these inks. But for watercolour, it works well, even for heavy washes. So I'd say that you can discard the "light washes" description. It can withstand heavy washes.
Lastly, as you can see, the paper warps a little when a lot of ink is used on the page. But not a whole lot.
The only downside of this book is that it is a little heavy. It weighs 444g. Carrying this sketchbook in my sling bag when I'm outdoors sometimes strain my shoulders. But if you don't mind carrying heavy bags, not a problem.
All in all, I like this sketchbook for the fact that it can take both dry media and inks. I seldom use dry media in it but I've been drawing a lot in it using my dip pens and various fountain pens. It just feels great drawing in this sketchbook.
For US$15+ on Amazon, I think it's worth it.
Try searching on Jackson's Art Supplies (UK) also.