The Zeta series of sketchbook belongs to the extra heavyweight line of Stillman & Birn sketchbooks.
There are 52 pages of 270GSM paper with smooth white surface. Paper is acid free so it's archival quality. The black hardcover is thick with faux leather texture.
The medium most suitable for use on the Zeta are ink, markers and colour pencils.
Mine's the hardbound A5 size. The pages can be opened flat so that's good for drawing big and scanning. The Zeta is also available in wire-bound in various sizes from A5 upwards.
Fountain pens and technical pens (such as the Rotring Isographs) run across the surface smoothly. Heavy washes of black ink from brushes are also handled well without feathering.
Markers work very well and are my favourite medium for this sketchbook. The thick paper absorbs markers without bleeding through. For really wet markers, you do notice a faint impression of the marker stroke from the previous page, but it's not nothing to worry about, but it's something to take note of. I've showed my sketchbook to two friends and they commented that the marker drawings look like they were printed.
About marker ink bleeding through, if your markers are really wet, then they will definitely bleed through, such as the red marker that I used on the previous page. Generally speaking, the Copic markers that I use don't have this sort of problem. The trick is to draw fast with markers.
The paper is thick enough to handle heavy washes. The surface is smooth so it works sort of like hot press watercolour paper. Since it's not cold press, you won't have the texture showing beneath your washes. Some people prefer like that textured look when they scan their watercolour work. Personally, I prefer cold press paper and Stillman & Birn does have that for their Delta series sketchbook.
With care, you can create flat even washes.
Another reason for the patchy look is because the paper isn't 100% cotton, so it doesn't absorb water that fast, and the water tend to stay and glide on the surface which when dried looks patchy.
I did not try any pencils but I don't expect any problems with the sketchbook. I'll update this review when I do. For mainly pencils or colour pencils work, I recommend the 150GSM Epsilon series sketchbooks instead since you don't need such thick paper and there are more pages.
The Zeta series sketchbook is recommended for those who use a lot of ink and markers, some pencils, and maybe watercolour if you like the smooth surface to paint on.