I have not read Lingua Franca so can't recommend a Sanskrit counterpart. It is hard to make a recommendation without knowing what your skill level is. You can go to the below bookstore and find something appropriate for your level. On the right there's a drop down 'Select a Catalogue' which shows different categories of books. I would recommend buying contemporary literature instead of ancient. Ancient literature is very obscure and difficult to understand. Many of the books on the below store are written in the last few years and they use much easier language. But keep in mind, virtually all the books on this bookstore have 100% Sanskrit with no translation. So you have to know at least some Sanskrit before being able to use any of them.

https://secure.samskritabharatiusa.org/np/clients/sb594/gift...

If you don't know any Sanskrit below are couple of options -

https://www.amazon.com/Cambridge-Introduction-Sanskrit-M-Rup...

https://www.amazon.com/Introduction-Sanskrit-Part-Thomas-Ege...