How Many Free Books Should You Give Away To Help With Book Marketing?

I made the mistake many years ago, with another ebook of mine, by not offering it for free download straightaway. Initially, Game Play, was priced at around $1.99 when it was launched in early November but I only had 3 or 4 sales. After reading how a good marketing strategy would be to offer at least your first book for free, I changed the price.

Now Game Play is free on Lulu, itunes and Kobo. It is also listed at Freebooks.net where the download count is currently at 851 in 11 days. The other site that is doing well is iTunes at 61 downloads since 24th Nov. 

My problem now is that Sing The Blues has not received much interest at all and that includes requests for reviews. I am wondering if writing another book (maybe shorter or about something else) and giving that away for free would help sales to that book?