A book for learning Ubuntu [duplicate]

Since I've just started to use Ubuntu, can you recommend me a good book or anything very useful in pdf format? I have version 18.04, and I would not want to constantly search everywhere on the Internet whenever I have troubles with commands, settings, instructions and stuff like that, but that the most important be in one place, like a good guidelines?

Thanks :)

You can see an online manual below:

Ubuntu Desktop Guide

and from the same site here, you can download a pdf (at the moment) for 16.04 (and other versions) but I'm sure before long there will be a pdf for version 18.04.n and whilst there will inevitably be some differences between versions 16 and 18, this should provide you with a reasonable grounding in the basics until the v 18 resource is made available.