Very interesting question. I am in a somewhat similar situation, although I have not written a book and I couldn't if I tryed. I have had hundreds of old farm tractors through the years. I guess that qualifies me as a good person to ask. I also put a website together years ago showing a lot of my collection. Got to a point where I received so many emails, especially about values that I literally took 7/8 of the website down.
What I found interesting through the years is many years back, you could actually get into some very interesting conversations, even make some lifelong friends with the give and take and you could quite often make a difference. Years ago I always considered it a "karma thing" and I took all the time needed to answer questions as thoroughly as I could. As the years progressed I had less & less time for it all. But I also noticed through the years exactly what you mentioned. It seems with the age of cell phones and such that can be accessed within a moments notice I guess, I seem to get a ton of emails asking me "what's this worth" in the subject line and not so much as a photo to accompany it, then no text or message telling me anything about the item. My rule of thumb anymore is I put as much into my reply as they put into the message. If I get a message saying "what's it worth, I simply reply I'm not at all sure. But when I get emails asking a value, with a well thought out message about where they are located and what the condition is and details such as if the tractors run or not or how many implements are with it and a million other things to consider including what has and hasn't sold in the recent past and rarity etc. In other words, a well thought out message gets a well thought out response. That's the kind of email I'll spend as long as it takes. But I always give a disclaimer because I'm far from an expert. Values are so hard to deal with, even for experts. What something sells for today may not come close to that tomorrow. Certain things are worth more to certain folks, even distance has a great deal to do with purchases anymore. So values can be very hard to pin down in this day and age. Too many scenario's to consider.
I've seen a ton of folks take advantage in certain situations where perhaps someone older or younger could have used a little good advice. Happy to say I've helped some of those folks out, sad to say I'm sure I missed some along the way. The most interesting thing to me these days though is probably 9 out of 10 emails are quick one line what's it worth emails where 20 yrs back it was the other way around. I also frequently get plenty of emails with pictures of a very common old tractor that might sell for a couple hundred and they tell me how extremely rare it is and they'll sell it for a couple thousand :-) I have never bought a tractor from someone asking me advice though, just seems like a conflict of interest in my humble opinion.
Something I get a huge kick out of...and this happens alot anymore! I'll be watching an interesting old tractor on Ebay or elsewhere, see it sell, and literally within minutes get an email from the new owner asking the value! Right after he placed the winning bid :-)
One quick great story, makes it all worthwhile....I saw a post where someone had advertised a very nice somewhat rare tractor, a fair value at the time was between 5-6000. It was advertised for $500. I emailed, not to buy the tractor, but to ask if they realized there was a zero missing. Turned out to be a lady who's husband had passed and not left her with any info so she did not have a clue. She changed the value to 5000 and sold the tractor an hour later for her asking price and both seller and buyer got a great deal. She was from Canada, a few weeks later, somehow she tracked my address down, sent me down a goodie basket with all sorts of Canadian treats, boy did that make me feel good that I was able to help her. Some good things about being in this position, and some bad, but always interesting...
Wow did this get long! Sorry...