Recording Spell Formula

I have a player who wants to record spells on his skin with Craft (Tattooing).
Can spells be recorded on places besides a spell book?

A fairly arsey observation about your players desire - A spell book can be taken away, lost, stolen, burned, eaten by acid, soaked, or otherwise damaged in a thousand different and interesting ways. This is a method by which an element of a) story and b) play-balance can be given to Wizard characters.
A character having their spell-book tattooed onto their skin avoids many/most/all of those risks to their spell book. It becomes very difficult to put them in a situation where they cannot have access to their spells. Personally I’d disallow it because, well, it’s just a bit too powergamey for my taste.
Alternatively - a spellbook has 100 pages. The accessible parts of your skin probably amount to about 10.
Also, a Wizard needs to keep ‘tweaking’ his formula based on the day, phase of the moon, star sign, location, spirit court, etc… etc… hence why spellbooks are described as journals, not encyclopedias. He’d need to make a Craft:Tattooing roll (or more than 1) every day in order to keep any tattooed spells ‘up to date’. Possibly.
Those latter 2 reasons would be sufficient in-game reasons for you to say no, if you were so inclined.

I’d say allow it with restrictions… A limited number of spells can be stored there, and injury risks making a spell unreadable. Make him specify where the spells are, with approximately 1 page worth of space in a spot on his body he can see per spell level.

James: I think you’re right about the practical considerations that would prevent this, but don’t underestimate the ways that one’s skin could be “taken away, lost, stolen, burned, eaten by acid… or otherwise damaged.” Especially if it’s the medium for rare and valuable inscriptions.

“Pa, watcha do’in ?”
“Sharpenin’ my blade, gonna skin me a mage!”
Honestly, it’s like having a big target painted on your back. Mwa ha Haah.

Charlatan: Well, yes… admittedly I was in a grumpy mood when I wrote it, but there’s always healing magics.
Sean: shudder

I would say that you could record spell formula on your body as tattoos, but you couldn’t maintain your body as your actual spellbook.
But you could copy the formula from other people’s spellbooks onto your body and then burn their book. Later when you want to put that spell into your repertoire you learn it from your tattoo.
I would also say that you could tattoo scrolls onto your body. Say one scroll per half-limb (shin, forearm, etc). When you cast the scroll the ink fades away. The tradeoff is that you can’t lose the scroll, but you also can’t give it to a friend to use, so I wouldn’t even charge more.

On this subject… the Auran Empire has (in various iterations through the years) used tattooed “runic” magic items to great effect. E.g. instead of gauntlets of ogre power your hands are tattooed with runes of ogre power. The trade-off is, again, flexibility v. permanence. We also used bodypierced rings of spell storing. The original reason blade-dancers wore little armor was because they were assumed to get runic tattoos over time and it was a mark of power/awesomeness to reveal them to the foe.

Great. Thanks for the advice everyone.