The option to 'Send to Kindle' doesn't appear under 'Send to' button fro these two titles 'Head First C' or 'Making Things Talk' but is ok with my other titles.
Any advise or reason why, please?
Unfortunately Head First C is larger than the limit Amazon has set of 50MB for sending to Kindle. You can transfer larger ebooks to your Kindle by downloading them to your computer, connecting your Kindle to your computer with a USB cord, and moving the file into the Kindle's documents folder.
The 1st edition of Making Things Talk is only available as a PDF, and the Send to Kindle option is only available for the mobi format. In the "Your Products" section of your account on oreilly.com should have the option of getting 50% off the new edition of the ebook (which you can use the send to Kindle option with). Thanks for your interest in O'Reilly ebooks and have a great day!
Customer Service Representative