Apparently, the reason for the 'Mech pictures and advertisements at the end of many of the novels is due to the cost savings when your book's page count is divisible by eight. (Printers print in 'signatures' of 8, 16 or 32 pages, and it costs more if you have a non-standard number of pages.) So to reach the desired number of pages, filler pages with art and ads get added.
From:
http://www.writing-world.com/publish/preparing.shtml"Page Count: Once you've formatted your book according to the template you've designed, you'll be able to determine the final page count. Keep in mind that offset-press (as opposed to POD) books are printed in folios (units of 8 or 16 pages). Thus, the actual printed page count of your book will be divisible by eight. Blank pages will be added to the end of your book, as needed, to complete the last folio.
You can save money by reducing the number of folios in your book. If, for example, you're just a few pages "over" a multiple of eight, take a look at your formatted manuscript for places where you might be able to "lose" a page. Do you have a chapter or two with just a single paragraph "hanging over" onto a right-hand page? By trimming each chapter so that it ends on a left-hand page, you can save not just one page but two.
You can reduce the number of pages by reducing your overall font size, but be cautious. Experiment with different fonts to find one that is readable in a smaller size. (For example, Century Schoolbook is more readable at 11 points than Times is at 12, and will save you a bit of space.) Don't arbitrarily reduce font sizes or tighten line spacing just to save pages, as you'll be doing so at the expense of readability.
If you still need blank pages at the end of your book, see if you can use them to promote another book (e.g., by including a brief excerpt). You might also consider swapping "ad space" with another self-published author who has related titles."