I currently don't work with a writing mentor, and was wondering if I should worry about covering the same things in multiple books on the same subject for self help. I'm basically going through the Catechism writing on a subject.
Here are things to consider while deciding whether or not to repeat information:
How much overlap is there between the audiences of the two books?
If your two books are Cooking for College Students and Cooking for a Family of Five, then you should assuredly repeat the information, because most people only belong in one of those audiences, and therefore will only read one book.
On the other hand, if your two books are Cooking Chicken and Cooking Beef, then there is a much higher chance that one person will read both books.
How fixed is the reading order?
On the other hand, even if your two books are in fact Cooking Chicken and Cooking Beef, you can't be assured that a reader will in fact read both of them, and certainly can't be assured that they will read the chicken book first. On the other hand, if your two books are Cooking Simple Chicken Dishes and Cooking Advanced Chicken Dishes then readers are far more likely to pick up the first book before acquiring the second.
How important is the information?
Some information is critical to readers' understanding. Some is not. Most falls somewhere in the middle. Assume the worst case scenario - your reader picks up the second book without knowing the first book exists. (This will happen - even if they're labeled volume 1 and volume 2). What happens? Do they die of food poisoning because they tried to cook their chicken medium rare? Or do they miss one cool trick to quickly debone a chicken? One of those outcomes is disastrous - the other is minor.
How much space does the information take up?
If it's only a paragraph, then you might as well include it regardless. Worst case is that some people spend an extra minute or two reading things they already know. On the other hand if it's five chapters then maybe leave the redundant copy out of the second book.
Alternatively, summarize and reference
If the repeated information is long, but too important to leave out entirely, then consider summarizing and then referencing your other book.
Step 4: Debone the chicken. For more information on quickly deboning a chicken, see Cooking Simple Chicken Dishes, chapter 2.
Here you get the best of both worlds - you give your readers the information they need without taking up too much space, and instruct them on how to find the rest of the information. As a bonus, it serves as a plug for your other book and might gain you a few extra purchases.
Well, it depends on exactly how much information is being repeated in your other books. For the most part, readers will want new information in the book they purchased. That doesn't mean you can't have any old information. One way would be to acknowledge and build on the idea that was in the last book. For example, if it is a self-help series, you could start a chapter by saying "As said in book _ you could achieve this by doing that..."
If you are worried about not repeating enough information, you shouldn't really be worried. However, it could be helpful to a reader to relay some of the most important key points from the last book, as long as you include a majority of new information.
Hope this helps! :)