Although some authors make their books available to read online for free, I'm afraid this is not one of them.
Manian Magee is Jerry Spinelli's most successful book, so it's likely that he is dependant on the sales of this book as a source of income.
However I have found that iTunes are currently allowing users to download free samples of the book, so you can at least see if you like it before buying it.
There are also ways in which you can buy the book cheaply, for example if you buy the book second hand, the story will still be the same, it just might not look brand new.
You can purchase second hand books from a number of places. The first place that it is worth looking is charity shops, or even better, charity book shops. There is usually a charity shop or two in most high streets, and they usually have a wide selection of books.
It is worth popping into these shops to see if they have Maniac Magee.
If you have no luck finding the book this way, then have a look on ebay.
Ebay will often have a "buy it now" option for books with cheap postage. So if you search for Maniac Magee under the book category of the site, then chances are you could pick the book up for £2/£3.
Another website worth looking at it is Amazon. At the moment the site is selling a used paperback version of the book for as little as 1p! Or you could splash out and get a new hardcover copy for only £5.99.
There are lots of options out there on how to get this book cheaply, if you put in a simple search of 'second hand Maniac Magee' into a search engine then a lot of options of sites to buy the book cheaply should come up.
I hope this helps.