I'm sorry to have to say this, but I'm not sure I believe in the actual, factual existence of Anna anymore. At the end of MGTIA, Fynn tears up Anna's grave marker and throws it away, saying it "wasn't necessary." So if you never find the place, it's because he removed all sign of it--not because she was only a literary vehicle he used to project his ideas and personal philosophy.
And then there's this very short collection of scraps of Anna's supposed writings. Except that Anna is meant to have been a very small girl, from age 4-almost 8. Yes, she is meant to have been numerically gifted, and able to see and understand patterns and relationships between things like light and shape etc. That's fine. But the writings are sparse (never mind those "several" boxfuls she was said to have generated) and fall into the all too common trap of being able to write complex words such as "orchard" correctly, while short words like "have" are misspelled.
Then there's the dis-accord of the writings themselves. At one point Fynn tells the reader that "Mummy" is an imaginary mother-figure Anna made up, an idealised mother. Yet again and again, "Mummy" obviously refers to Fynn's actual mother. There are the rather too numerous panegyrics to Fynn himself; over and over, page after page of "Fynn is so special and wonderful and I love him and want to marry him and have babies with him." (Ew). No references to the other members of the household (such as Fynn's brother Stan), her pets, or friends.
Even Anna's "Mr God stories" don't ring true if you've read the first book as often as I have over the years. They don't chime with the way we were first told Anna thought about God/Jesus/the Holy Spirit, particularly the latter, who has somehow turned into a schoolmaster who gives her lessons on laziness etc.
Then of course there's the 12-year gap in publications; MGTIA came out in 1974, while Anna's book didn't see the light of day until 1986. Odd, that, given the immense popularity of the first volume; if Fynn actually had all these fragments sitting around, I should think he would have produced the volume sooner.
No, sorry, not convinced. I almost wish I hadn't read it. We all hate to have our perfect little dreams shattered in the cold light of day.