These may be a worthless comments or fall on deaf ears but I personally feel need to be made. Some things for readers to keep in mind:
1) Taking time in hours and expenses into account, I know of few authors (if any) who profit financially from writing. Those who send letters requesting piles of data and material need to remember they are probably not the only people who write to an individual. My own postage bills are a 4 figure sum yearly. Include postage costs as a common courtesy. Xeroxes of documents and prints do cost. I had an author friend get a note by with simply "Send me copies of all the photos and documents you have on "Wiking." Sender didn't even enclose a stamp.
2) Books are written based on available material, not by desired topic in some cases. With documentation gaps, a lot of areas can not be covered in a book format. I often get requests or questions as to why this or that topic doesn't have a book: its because enough info often doesn't exist.
3) Non-writing readers/collectors often have material of use but, for reasons I've never understood, don't take the effort to share it. A lot can be added to a lot of projects from this group. A single photo, document, or period book adds a lot. Location of readers can assist authors if close to archives used, museums, etc. The more resources someone has the better. Most writers have a small group who assist and archives as sources with little contributed from the general reader who may have something of use be it whatever type of material. The more help, the better the project.
4) Its impossible to do books with 100% new data, even if only to give needed background and relative data to a reader (especially new ones). I'll personally buy a book for a single photo or a couple pages of new information. A book with 20% new data is (to me) a must have gold mine.
5) A lot of books are passed on by US readers due to language. However, many have a lot of data that can be used with a minimum of foreign language ability. Books like the Biblio KC series, Tessin, and MANY other titles should be considered by those who are not fluent. A number of good and inexpensive military dictionaries are also available. Dates and MANY other facts require almost no foreign language skills. The average person has a 1200 word vocabulary, so if you can recognize a couple hundred words in another langauge there is a lot that can be absorbed.
6) I see the junk "bargain table" books that continue to be cranked out appear now even in good quality bookstores as WWII is popular. Most all know the titles or authors, they've obviously been written in a day, no bibliography/footnotes, grainy pics and/or facts totally copied from others, superficial (or totally wrong) info, etc. Wasted money is wasted money but many books sell simply due to the distribution network of a large publisher that floods the market. If someone on this forum seeks a book on a topic and doesn't know what to buy, ask for suggestions. There are far more good books than bad, the better ones should be supported while the "production line" type should stop wasting paper resources. You get what you pay for.
7) Authors do not decide the cost of a book. Publishers do. Likewise, few authors determine which photo is done large, which is done small. A small number allow author design input, most don't. Few publishers have a clue about the topic compared to the author, a factor to consider when complaining about this or that aspect in the finished product.
If you get a book and like it, invest 2 stamps in telling BOTH the publisher and author. It gives the author some reader insight for future projects and motivates the publisher to do another volume on the topic by the author. Publishers are in business to make a profit and only do so with satisified customers.
9) This one will irk a few. I see some readers on forums who make a career of blasting this or that mistake in any book, with no positive comments even when deserved. I have never seen a perfect book in any language with zero mistakes of any type (design, a fact, caption, ID, typos, etc), including my own. To that small fraction of "blasting only" readers: Since your knowledge and perfection level is so high, please let us all read the originally researched book YOU'VE written.