References for questions on Arabic linguistics, relevant to Quran and Hadith.
Arabic-English Lexicon by Edward William Lane
This is an 8 volume lexicon produced in the 1800's that draws upon various medieval arabic dictionaries. Available on archive.org as a scanned book.
Tyndale archive has the ability to navigate it via alphabetic list of catchwords (like a dictionary). Searchable through arabic text at yulghun.com
References for the consensus of Islamic schools of thought on various religious
duties and laws.
Kitab ar-Risala fi Usul al-Fiqh by Imam Shafi
Foundational text on the principles of fiqh, written by Imam Shafi, founder of the Shafi school of thought. Available at archive.org.
Minhaj Al Talibin by Imam Nawawi
The book is a summarized compendium of Shafi jurispudence. Available on archive.org.
Al-Hidayah fi Sharh Bidayat al-Mubtadi by Burhan al-Din al-Marghinani
A compendium of Hanafi jurisprudence. There are two translations of this work available. An Old tranalation commissioned by the British Empire and done by Charles Hamilton which covers pretty much the whole work, but excludes sections on acts of worship, war, criminal laws and slavery etc. A new translation by Imran Ahsan Khan Nyazee that is not complete yet but covers some parts missed in the original translation.
Risalah of Ibn Abi Zayd by ibn Abi Zayd al-Qayarawani.
One of the earliest books in Maliki fiqh, especially fiqh of worships.
Furthermore one may find books on Maliki fiqh on Muwatta.com site which has scans of Muqadimat al Izziyyah
Fiqh ala al-Madhahib al-Arba'ah by Sheikh Abd al-Rahman al-Jaziri
"Islamic Jurisprudence According To The Four Schools" is the translation of a fairly recent work that compiles the rulings of the four Sunni schools on acts of worship such as prayer, purity, fasting, pilgrimage etc. Available on archive.org
Reference Books on Islamic History, specifically on the events of the lives
of the Prophet and the Rashidun caliphs.
Seerah Rasul-Allah by Ibn Ishaq
One of the oldest available biographies of the Prophet. The original work has not survived but is reconstructed from derivative works, especially from Ibn Hisham and Tabari. Translation by Alfred Guillaume available on archive.org
A single volume of Tafsir Qurtubi is also available in an abridged form.