Update: July 31, 2012 - Recently, AstroSage.com has also published Hindi translation in Devnagari/ Unicode format online. Check it out - Lal Kitab in Hindi | लाल किताब हिन्दी में
A Hindi transliteration of 1952 edition of Lal Kitab (last and the most detailed edition of Lal Kitab) is available at Scribd at -
You can read it online or download it in PDF format.
1941 Edition of Lal Kitab is available in Hindi to download from -
Apart from this, two other editions of Lal Kitab books can be found in Lal Kitab Yahoogroup's file section (http://groups.yahoo.com/group/lalkitab/files/). Here are name of two files -
I would generally advise to buy the printed books rather than online books, as now a days good transliterations are available in Hindi. Most of the online books are having too many errors or not good in terms of quality. Lal Kitab 1941 book mentioned above is an exception which is as good as the print edition of 1941 edition.