Do you know any free available books related to GIS ?
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Last weekend, Slashdot informed us The National Academies Press are offering all their books, over 4,000 of them, for free in pdf format. With searches, you find plenty of geospatial-related books, for instance:
ESRI has a lot of technical papers and books specific to their software availible here. The documents are in pdf format and availible for download.
A practical guide to geostatistical mapping: http://www.lulu.com/product/paperback/a-practical-guide-to-geostatistical-mapping/6379056
And of course many open source gis programs have their manuals online, eg:
Google Books with the "full view" filter applied lists some 38,000 titles for "geographic information systems", 31k if you leave out magazines. Of course there's a lot of irrelevant stuff in there, but with judicious use of extra keywords good ones can be pulled out. When you find good ones they can be saved in "my library", which can have various sections (favourites, to read, GIS, etc.). If desired, you can export your library as xml. Sharing a library could be more straightforward, url wise (I think it should be -http://www.google.com/profiles/johndoe/books), but basically you just copy and paste the obfuscated address bar contents; here's my favourite. Yes, there's only one. :) Although I'm fascinated with Google Books and think it's a great idea, I don't like reading long works online.
I just came across the Quantitative Methods Research Group (QMRG) of the Royal Geographical Society which has a series of short methods articles (CATMOG).
These are all papers detailing some type of statistical analysis of geographic data (although many don't have a strict relationship to spatial data). From the ones I've skimmed through and read, they are pretty gentle and brief intro's to the topic at hand. Although I don't doubt maybe more exposure to statistics than a university level intro course is needed.
Also if one is interested in regression with spatial weights you should definately check out the Geoda workbook. I also know of another short intro pdf to spatial regression, but I have not had the chance to read it yet (Ward and Gleditsch, 2007).
The NCGIA Core Curriculum in GIScience is still worth studying, even though it's now over a decade old and was never completed. It is well organized and written by luminaries in the field (Reg Golledge, David Unwin, Tim Nyerges, Michael Goodchild, Robert Haining, and many more). Altogether it's probably book length, but it reads like an extremely detailed outline, making it easy to find what you are looking for.