Well, they're not numbered, so I may be making assumptions, but I always assumed
'muderous maths' was the first
and 'more murderous maths' the second!
'The Phantom X' - algebra
and 'The fiendish angletron' - trig
are probably the hardest and best avoided at least until after the others. Ds came back to the trig one a year or two after having read, but not really followed it first time around.
'the perfect sausage' is also quite hard (it covers circle theorems areas & volumes & stuff and touches on trig too I think, but in a 'push these buttons on your calculator' kind of way, rather than actually explaining it).
'vicious circles & other savage shapes' would be easier than the sausage one lots on triangles & stuff in it.
If she's good at and interested in maths & enjoys the first two then:
'The essential arithmetics'
'Do you feel lucky'
'Numbers the key to the Universe'
and 'The Mean & Vulgar bits'
would probably be the next ones in difficulty(ish) after the first two. (I'm guessing a bit though).
There are some puzzle books too.
Ds was another would read anything & everything type child & started getting these out of the library & reading them in about year 3 (but he's also very good at and very interested in maths). In about year 5 I saw the whole set on discount somewhere (probably the book people) & as I'd noticed him seeking them out at the library repeatedly I bought the lot. Not sure really if his abilities in maths stem from his obsession with these books or vice-versa - it may well be a bit of both (he was good at maths since before he started on the books, but now he's very good at it iyswim).