https://github.com/gigaherz/Guidebook/blob/master/examples/config/books/demobook.xml

in it you have

<img src="domain:item" />

however, the mod wants the <img> tag to be terminated with </img> and not />

Good to know, because I use one of java's most used XML libraries, and if it didn't work it would mean the lib is broken. ;P

sorry this was caused by invalid xml on my side it does want />