A recipe from an older cookbook often reads as a list of ingredients followed by a sentence. The instructions are not careless; they were written for a reader who already possessed the knowledge that modern recipes have to supply.
Technique was assumed, not explained
Cooking was learned by watching, and a written recipe served as a reminder for someone who had already been shown the method by another cook.
So a nineteenth-century instruction to make a sauce might specify quantities and then say to proceed in the usual way, because there was one usual way and everyone reading knew it.
Modern recipes carry the opposite assumption, and most of their length is technique the older book left to the reader's background.
The equipment behaved differently
Solid-fuel ranges had zones of different intensity rather than a dial, so recipes spoke of a slow oven or a quick oven and expected the cook to find the right spot.
Times written for those conditions do not transfer to a thermostat-controlled oven, and neither do the pan sizes, since standard tin dimensions have shifted repeatedly.
Something as ordinary as a cup varied by country and by era, so a volume measurement copied literally can be meaningfully wrong.
Ingredients are not the same materials
Flour protein content, the strength of leavening agents, the fat content of cream and the size of a standard egg have all changed within living memory.
Vegetables and fruit have been bred toward sweetness, uniformity and shelf life, which alters both cooking time and the amount of sugar a recipe needs.
Meat is generally leaner, so old braising times that assumed abundant fat and connective tissue now produce dry results in the same number of hours.
Preservation was doing structural work
Recipes written before reliable refrigeration used sugar, salt, acid and fat in quantities that look excessive now, because those quantities were keeping the food safe.
Reducing them in a modern kitchen is usually reasonable, but it changes texture as well as flavour, since sugar and salt are structural in preserves and cured items.
This is why old preserving recipes should be treated with particular care rather than adapted casually, as the ratios were doing more than seasoning.
Reading them is still worth the trouble
Older books record dishes that fell out of fashion and preparations built around ingredients that were once ordinary and are now unfamiliar.
They also show how a cuisine actually ate, rather than how it presented itself, since these were working documents rather than published aspirations.
The practical approach is to treat the old text as a description of an outcome and to supply the method from current knowledge of how that outcome is reached.