They don't seem to be giving the correct instructions in concocting the potions- as seen when Hermione isn't able to make her draught in Slughorn's class.
I would hazard a guess that potion making is insanely more tough than cooking. Remember, this is something that is taught as a full subject over the course of (at least) 5 years.
There once was a course that I had to do, Engineering Drawing (Technical Drawing). It was about drawing things using a robust protocol and tools like a drafter. It seemed quite mechanical and easy to me -- all we had to do was make lines in a certain order.
Except that it wasn't easy. While it was easy to figure out what lines had to be drawn, we all made infinite mistakes. You make a mistake in one line, and notice it 20 lines later -- you have to redo the whole thing.
I think that Potions is similar. While there's a definite set and order of things to be done, these are easy to mess up. One mistake, and the potion will stop working. Following the instructions correctly may not be as easy as it looks. Hermione was never portrayed as infallible; it's quite possible she made a mistake.
Also, take a look at cooking. While following the instructions usually gets you an OK product, there's always something that a skilled cook can tweak to make it better. And there's always something a newbie can mess up. Same goes for Potions.
Here's another thing, as mentioned by Rob below: Not all the recipes may be exact. There might be something akin to "season to taste" found in cookbooks, or "left as an exercise to the reader" found in math and physics textbooks.
While I don't disagree with Manishearth; I always thought of it like the old saying:
"Those who can't do - teach." (1)
But amend it with
"Those who can't teach - write the text book."
Most of potion recipes were passed down through the ages; there might have been reluctance to alter them.
It is also entirely possible that the authors of the books were not as gifted in potion-making as Snape was (e.g., to crush the bean with the flat side of a silver blade instead of cutting), and did not think outside the box.
When it would be time to release a new version of potion-making recipes they would add new ones, but leave the old recipes alone; thinking "If it isn't broke - don't fix it"
(1) Not to say that the teachers at Hogwarts were not exceptional.
The potions books are not actually misleading, and Hermione can make the draught, it just takes longer than when Harry makes it with the help of Snape's side-notes. The reason for that is: Potion recipes are like regular food recipes, while the recipe is the same, the potions masters (just like the cooking chefs) don't follow the recipes and do something in a way to make the recipe faster or more powerful. For example, in one of the side-notes, Snape says that crushing an ingredient with the side of the knife is more efficient than cutting it, which makes Harry go through that point of the recipe faster, while Hermione, who follows the original recipe, uses a less efficient method, which leads to taking longer to complete a step.
So, again, the text books are not misleading, they just don't have, necessarily, the best way to do something.
Hermione is able to make her draught, and Slughorn approved of it, as is clear from the relevant passage in Half-Blood Prince.
Hermione's potion he gave an approving nod.
It's just not perfect. This is to be expected, even of someone who is a decent potions-maker and follows the textbook. As Slughorn tells them when they start:
We have a little over an hour left to us, which should be time for you to make a decent attempt at the Draught of Living Death. I know it is more complex than anything you have attempted before, and I do not expect a perfect potion from anybody.
Thus, the textbook is not inherently misleading. It's just that under the specific circumstances even someone as good as Hermione can't get a perfect potion by following the book.