Here I'm sharing some of my experiences and tips with gesso.
What is gesso?
Gesso's original use was to prime a canvas, it is in a way a superior quality of the paint you use on walls but normally it is thinner, It makes the surface more absorbent so the paint binds better but not too absorbent so it sucks up all the paint. It can be painted on a lot of different surfaces and can be bought in different grades from very cheap hobby quality, to student and artist's quality.
How to use gesso?
From my experience the thin gesso is best in a sketchbook as it doesn't make too much of a brush texture. So if you are thinking about using it ask before you buy it, or it maybe a good idea to thin it with water before use but be careful when doing this so you don't get it too thin, better to put in a little water at a time.
Using gesso makes the page heavier and probably won't work on thin flimsy pages so it is a good idea to test it out before you start.
I always put a piece of paper under the sheet I'm painting to protect both the empty pages or the work underneath. If you are using coloured gesso please be careful that it doesn't go onto other pages if the book is stitched in the middle (not all sketchbooks have this problem). I have tried this with black gesso. Above is an example of what I mean, I've used another medium than gesso but you can clearly see the pink bleed through the stitching. Fortunately I can correct this by applying more paint but it could well have been a drawing on the pages where it wouldn't have looked good. I started with a layer of white gesso on these sides, working with watercolour and gouache on top together with Posca pen.
One good tip is to use gesso on pages that might bleed through, so if you have bought a sketchbook that doesn't live up to your expectations this might be a solution. I use it on my Moleskine sketchbook. Gesso can be used to great effect too. The advantage of gesso is that it has a mat finish and doesn't stick with the other page when closed. I've used black gesso from Golden Acrylics here as a background to enhance a picture I'm glueing in. Tip - use other types of paper and stick them into the book if you like to use different media like me.
Here is an example of a page with black gesso underneath that was worked upon with coloured pencil and Posca marker.
Here are a couple of sides where I have used the black gesso in parts of the right side, this is not a side I'm happy with but decided to post it anyway to show that it can be used as paint in a sketch as well as underneath. On the page on the left side I've stuck in two Gelli plate prints.
You can also mix gesso with watercolour to give a pastel effect that is permanent and worked onto, or mix gesso with a little acrylic colour.
Finally that awful mistake you are so upset about can be covered with gesso so you can start again.