I solicited and received such advice on the mailing list and received a number of replies which are all listed verbatim in a separate memo: "Advice to Parents who Discover their Child is autistic" (URL: http://www.autism-resources.com/advice-to-parents.html). This is a very brief summary of some of the items:
Michael Powers's Children With Autism: A Parent's Guide has a good reputation, but there are some newer books aimed at parents. Another book with a good reputation is Elizabeth K. Gerlach's Autism Treatment Guide from Four Leaf Press. There are a number of books describing parents' experiences with autism and though some may be slanted a particular kind of treatment, probably all have value in allowing you to share the experience of other parents. Catherine Maurice's Let Me Hear Your Voice is one example.
Every treatment for autism has its detractors and none has proven to benefit every case. Thus, the task of judging the effectiveness of potential treatments will ultimately fall on you--to a larger extent than you will feel qualified to make. If the professional to whom you take your child strongly recommends some program or treatment, know that there are others who will recommend some other just as strongly. Note that many programs are made up from parts of several methods.
In the USA, the national society is the Autism Society of America (ASA). It has local and state chapters, so it may help you to find a local support group. Another organization of value is the Autism Research Institute (ARI), which includes publications.