Basic information on musical instruments and makers is available in many of the sources listed under General Music Resources; Grove Music Online is particularly useful.
For further help with images and descriptions of musical instruments, see MIMO (Musical Instrument Museums Online), a free online database containing records of instruments held by a large number of museums around the world. The site is searchable by instrument families and maker's name as well as by museum.
The Minim-UK resource, launched in October 2017, supplies information on c. 20,000 instruments held in British instrument collections, along with articles, videos and other useful materials.
The Musical Instruments Resource Network site supplies many further sources, as well as helpful information on identifiying and caring for historical instruments (including how to make decisions about playing condition).