Your updated description is now visible at amazon.com
I actually enjoy the convenience of having my solutions checked when I submit them
here to earn points rather than having to compare them to print solutions. What I do is print puzzles from the e-books as needed, solve them using a pencil, type the solutions into a text file as I go along and then submit a solution once a day by copy-pasting it into the box (rather than typing it directly into the box). That way I won't have to re-type a solution if something goes wrong (I can just re-copy-paste it from the text file), and I end up with a file containing all the correct solutions so there's no need to request them by e-mail. I discard the puzzle sheets when the solutions have been verified and am happy that there's no paperback taking up space in my bookcase.
But then I am the kind of user who prefers solving puzzles online to solving them on paper. From my point of view the only possible improvement would be if these collections were available here at calcudoku.org the way the extra puzzles are currently available to subscribers. You wouldn't get a PDF e-book when buying a collection; instead, the puzzles would be accessible for online solving whenever you were logged in to your calcudoku.org account. (I am merely fantasizing here, of course. The nice thing for you as a puzzle author would be that you wouldn't have to share the purchase price with publishers and retailers, but it is far from certain that there would be many takers of this option so it probably wouldn't be feasible to put it into practice.)
Most of those who started out as users (not necessarily subscribers) at calcudoku.org and then went on to order e-books through lulu.com (or receive the bonus e-books included with a subscription) probably aren't bothered that the e-books don't contain solutions. But buyers who don't have any prior acquaintance with calcudoku.org and just order the print edition from Amazon so they can enjoy an actual puzzle book in their comfy chair, far away from any computer, are probably expecting the miniature solutions at the back of the book that are usually included if you buy a puzzle book from a newsstand. Even if the solutions are available free of charge, the reviewer at Amazon may find it cumbersome to go to the computer and open a file every time she wants to check a solution rather that just look in the back of the book. It may of course be that she doesn't realize the solutions are free of charge because you don't
explicitly state so (even if it should be clear from the context), but she is unlikely to have made that mistake. I think her beef is entirely with the "usability" aspect and that she would have preferred it if the book had contained fewer puzzles to make room for miniature solutions. Also, if she is not familiar with
killer sudokus, she may be disappointed at being referred to online resources (however excellent they may be), and she may be extra reliant on having solutions at hand at all times to see if she got the concept right.
Maybe you should go "e-book only" in the future and abandon the paperback format since e-book buyers are less likely to have this kind of issue with the books (and you aren't making any money on the print editions anyway). Of course there are the three
very positive Amazon reviews of "Calcudoku, 101 Advanced Puzzles", which are a very nice kind of exposure. But maybe expectations are different when a book has "Sudoku" in the title?
Got too long-winded again, sorry