LIBRE OFFICE http://www.libre-office.theappcenter.com/
This is an entire suite of programs that virtually replaces Microsoft Office. Two applications in this suite will be used: the word processor (as the basic document) and the presentation application (for captioning pictures.)
This strangely-named program allows the capture of portions of the current screen, and the captured image may be saved or copied for further processing. It's an ideal way to capture specific images from websites that include pictures that you may want to include.
Another oddly-named program (“Irfan” is the name of the author) which provides simple, but powerful image processing, such as cropping, converting to gray scale, and balancing brightness and contrast.
FAST IMAGE RESIZER http://fast-image-resizer.en.softonic.com/
Provides the means to quickly and simply reduce the size of large images to manageable proportions for inserting in the book. With a simple drag and drop, it will re-size multiple images in seconds.
PDF FILL TOOLS http://www.pdfill.com/pdf_tools_free.html
This program allows you to merge several pdf files into one assembled file, as well as many other tools you may use to manipulate pdf files.
Although you may already have Adobe Acroreader for .pdf files, I much prefer Sumatra – it's extremely fast, and I use it as my default .pdf file reader.