Resources for composers are more readily available today than ever, mainly due to the miracle of the internet. Among the best are the following, many of which are available at no cost for online viewing and/or downloading:
Beethoven and His Nine Symphonies - the complete 1896 book by George Grove in PDF format on the IMSLP website.
Counterpoint - the 235 page, 1947 book by Walter Piston is available in many libraries and from Amazon.
Dictionary of Music and Musicians - the original 1878 work by George Grove, freely downloadable in various formats on the archive.org website.
Dolmetsch Online - includes an extensive online multilingual dictionary of music terminology as well as free courses in music theory and history.
Grand traité d'instrumentation et d'orchestration modernes, Op.10 - both the original (1843) and revised (1855) versions of Berlioz's classic but still very useful tome on instrumentation and orchestration on the IMSLP website. The site includes English and German translations.
Harmony - the 1987 5th edition of by Walter Piston's 592-page work is available from Amazon.
Instrumentation and Orchestration - this 528-page 1997 book by Alfred Blatter is available in many libraries and from Amazon.
International Music Score Library Project - (IMSLP) provides a vast number of freely downloadable public domain scores as well as some recordings and books. A truly outstanding resource.
Orchestration - The classic 1914 book by Cecil Forsythe is considerd by many to be the best on this topic. Freely downloadable in various formats on the archive.org website.
Orchestration - the 488-page, 1947 book by Walter Piston is widely regarded as one of the two best on this topic. Available in many libraries and from Amazon.
Principles of Orchestration - Nikolay Rimsky-Korsakov's classic 1910 work is available for viewing online along with audio files and also as a free download on the Project Gutenberg web site. Newer editions, including at least one with useful commentary, can also be found on the web.