Beth Sapra library, with grammar by Nestle and NT by Gwilliam.
Syriac Peshitta page, with 1905 Peshitta of the British & Foreign Bible Society, and the translations by Murdock and Etheridge.
Syriac books, site of Brigham Young University, mainly literature histories and manuscript catalogues, also Nöldekes grammar.
Syriac Studies Reference Library, of Brigham Young University and the Catholic University of America, many works.
Internet Archive search for Syriac, also try other keywords, syrisch, syriaque etc.
Various Syriac books in the Daily Hebrew blog