Wheat paste is an adhesive made by heating flour and water. It is an old adhesive - it has been used for at least 300 years (and almost certainly longer, I just can't find any earlier sources). It works as an adhesive because heating the gluten in the flour changes it so that it becomes sticky.
Wheat paste is one of the primary adhesives used in bookbinding. As described below, it is strong, reversible, cheap, and archivally stable. But only if made as described below! Other pastes, like craft paste and wallpaper paste are not acid-free, and will eventually damage the books that you use them on.
To make wheat paste for bookbinding, you need pastry flour. Cake flour, bread flour, and all purpose flour are not the same - they all have more gluten in them, which, in addition to making the resulting mixture more gel-like, also attracts insects to the finished book. Gluten is needed in the flour in some quantity - it is what makes the adhesive work - but the flour with the smallest possible quantity should be used, as the gluten also contributes to the eventual decay of the project.
Pastry flour can be found at many large grocery stores, as well as most health food stores. It can also be found in many better art supply stores, at a huge markup. Often the flour sold in art supply stores will have preservatives added, some of which affect the archival stability of the paste. Because of this, only buy flour at art supply stores that is guaranteed to be acid-free.
It is definitely worth the little extra trouble to get pastry flour. Flour is cheap, and if you are spending the time to do a bookbinding by hand, it seems worthwhile to use the best materials.
To make wheat paste, start with one part flour to five parts cold water. This amount of paste created will be about the same as the amount water used initially. Slowly add the flour to the water, stirring, breaking up all lumps. If you have a microwave, boil the mixture, stirring every 30 seconds. Just before it is about to boil over, take it out, stir, and boil again. Repeat twice. The paste is done when it changes from a milky white color to a translucent hue. If you are using a stove, bring the mixture to a boil very slowly. Use a non metalic pan - metal can cause the paper to decay. Boil for 30-60 seconds - the longer it is boiled, the thicker it will be.
Once the paste has been boiled, it can be used, although many people prefer to let it cool first, so that it thickens and is not so watery. This can be done either by spreading it out on a flat surface or by refrigerating the paste. Stir occasionally as it is cooling for more even consistency.
As the paste cools, it will thicken considerably. Store it in an airtight container when not in use. It will last 2-4 days, depending upon humidity and temperature. After that, it will become watery and unusable.
For thicker or thinner paste, vary the amount of flour. If the paste is too thick, after it has been cooked and cooled, it can be watered down. If it is too thin, it cannot be saved, and one should start over.
Evilrooster added the following suggestions regarding extending the shelf life of wheat paste: I have run across a number of different preservatives you can add to the paste to stop or slow yeast growing...alum and formaldehyde are difficult to come by. But one of my books recommends clove oil. I got some from an aromatherapist friend, and my wheat paste doesn't go yeasty so fast now. Cool.
And then, later: I've heard of are formaldehyde, alum, and clove oil. I've been using clove oil, and it works a treat. One or two drops in a batch of paste retards yeast growth for up to two weeks.