Is it a good idea to just stay on Lunar and switch spellbooks when necessary? Or just when I'm doing something like Slayer?

Not really in my opinion. Unless you train magic with the spells.

If you do the occasional alch or telly or whatever regular spellbook spell, than yeah.

i used to do it for awhile, its great for showing off.

 

i think its great for slayer but im not an expert with slayer, it will just save you the time of talking to the masters, and if you dont care about drops that much then lunar is great. if you care about drops then use normal magic, you can still bank the important drops with lunar then switch to normal and alch later.