Hmm, I know the old sage about books and covers but any book which purports to be about mains electrical installation and sports a picture of a UNI-T UT33C multimeter would have me running a mile.
Don’t get me wrong, it is probably a fine book, quite possibly just using a stock photo, and for typically less than 15€ the multimeter in question is undoubtedly decent value for money - but I just would not use it on the mains.
One thing you can safely say about almost all UNI-T meters is that their insides generally fail in multiple ways to be adequate to support the CAT safety standard claimed on their outsides.
While I own UNI-T meters it is hard to recommend them for mains use - officially they have the safety rating but when you open them they use components which could never meet it - worse you never quite know what you will get - apparently the ones destined for the German market actually do generally have the right components in but they are often missing in units destined for elsewhere, so a random eBay purchase might meet the spec but probably won’t.
If anyone wants to buy a meter which is probably safe for poking around the mains then the starting point is something like the Amprobe AM-510, a smidge under 80€ from Amazon.fr
28!?! - how many do you actually use?
I think that you might have managed it. 
I put 12 in the kitchen - two dedicated to fridge and extractor fan - I’m not (yet) feeling that was too few.