N.B. The email exchange below may have been edited, e.g. to remove content not essential to the main point(s) or to standardize English spelling/grammar.
I was very interested to read your page on Balto-Slavic languages, in particular the comment that Baltic languages preserve a number of Indo-European characteristics lost in other Indo-European languages. Is there anywhere I could find out more about these characteristics, please?
To understand what is being said "about these characteristics," you'll first have to know a significant amount about Proto-Indo-European, and then about Baltic (and other Indo-European) languages...
Wikipedia offers superficial background, but some, understandably, question its reliability: one must be able to evaluate what one reads there (some of it is quite good!), but doing so requires prior background.
Find a top-notch university library, and be prepared to spend much time therein. Start, perhaps, with 2-3 really fine encyclopedias; they can provide a bit of background knowledge (but only that). Later, you'll move on to scholarly books about Indo-European historical linguistics. (It would help if you can read French and German; other languages, too, but E/F/G are the biggies.)
Check contents/indices for "Proto-Indo-European," "Baltic," "Old Prussian," "Lithuanian," and "Latvian" a.k.a. "Lettish." (I omit the lesser Baltic languages here, as more is said about the big three, but the others are not irrelevant.) Then read... and take careful (!) notes, so that you'll be able to refer back later and piece together a comprehensible story from bits and pieces. (See also our Books Online..., and our Baltic Online language lessons...)
Scholarly books are likely to cite journal articles; so you may need to find copies of said journals...
You won't understand everything you read, by any means, but the more you read (studying carefully) the more you can learn. ...It's all quite doable, but it takes time and effort.