[tigen]

Some of these books describe obvious (or at least uncontroversial) things. Like one book describes issues with Celiacs eating wheat in a way that implies it's a general risk for everyone.

However diet can clearly affect the gut microbiome which is involved in development of various immune diseases (including allergies). Less clear how it might treat it after the fact, where evidence seems lacking.

Diet can clearly affect symptoms/complications. If someone has gut inflammation then different foods (or fasts) can be dramatically more or less compatible, and could potentially improve quality of life or stave off surgery. In general though, expecting too much from diet is likely to cause disease worsening compared to getting on the best drugs.