From a Facebook group where someone asked about important books in learning Arabic:
Learning Arabic consists of three steps:

1) Learning basic language skills (can take 1-3 years depending on student)

2) Studying Arabic as a science (can take 2-4 years depending on student)

3) Intensive Academic expertise/research (Lifetime)

The first step consists of one word: Practice.

Practice, Practice, Practice your reading, listening, speaking and writing skills as much and as often as you can to get to to fluency. It can take 2-3 years to obtain full fluency (or more) don’t despair. The key is consistency in practice, even if its only 1/2 hour everyday (although more is better).

Important books for this are:

– the Bayna Yadayk series (preferably with a teacher who can give you speaking and writing practice).

– My personal favorite for self study: a book called “Learning Arabic through the Islamic texts” by Syed Iqbal Zaheer.

– Qasas-ul-Nabiyīn lil-Atfāl for reading practice

– Suwarun min Hayātis-Sahābah for reading after Qasas.

– Other things you can use for practice include Arabic Youtube (like the Omar series, Arabic cartoons etc)

– al-Nahw al-Wādih

Do not cross over to the next stage until you have achieved at least 80% or so fluency. Ask around for additional resources for practice.

2) Studying Arabic as a science – this should be done with a teacher

If you are a natural at languages, any teacher who has studied these texts will suffice, otherwise a teacher skilled and fluent in these sciences is highly preferred (doesn’t have to be a specialist in the Arabic language though, some scholars of fiqh and hadith can also teach language very well).

There are 4 sciences – Nahw (grammar), Sarf (morphology), Balāgha (prose/rhetoric/style) and Adab (literature).

The most important books are:

Nahw:

– al-Ajrumiyyah (with a lot of basic I’rāb practice)

– Qatr-ul-Nadā (With I’rāb practice from the Qur’an)

– Qawa’id ul-I’rāb (with I’rāb practice from the Qur’an)

Alfiyyatu-ibnu Malik (With I’rāb practice from the poem that forms the core text of the book).

– Mughni al-Labīb

Sarf:

– Imam al-Taftazani’s explanation of Tasrīf ul-Izzī – (with some basic practice in I’lāl and Ibdāl)

– Lāmiyyatul-Af’āl – I prefer the sharh of the author’s son for the student – although the teacher can use others to reference from.

– Alfiyyatuibnu Mālik (with a lot more practice in I’lal/Ibdal etc)

– Taysīr-ul-I’lāl wal Ibdāl – can be studied on one’s own after the alfiyyah – excellent book for practice.

Balāghah:

– Durūs-ul-Balāghah – an easy, comprehensive introduction

– Al-Balāghatul-Wādihah – Important to learn Balaghah on a practical level.

– Sharh Mukhtasar Sa’d (or Mukhtasar-ul-Ma’āni) – Very important text to study, will really make the connection for you between logic and balaghah, as well as opening up advanced balagah texts for you.

Adab:

– Any text in Urūd/Qawāfi with a teacher.

– Sharh Mu’allaqāt al-Sab’ or Ashr

– Selected study from al-Mutanabbī

– Maqāmāt Al-Harīrī – extremely important for building vocabulary

– Al-Umdah – a comprehensive text on the sciences of poetry. I forgot the author but I think his nasab was al-Qazwīnī

3) Intensive Academic expertise/research – a specialist in the Arabic language will know way more than I do.

Nahw:

– Other explanations of the Alfiyyah such as Imam al-Ushmūnī’s, Abū Hayyan, al-Shātibī, Ibn-Aqīl.

– Explanations of Imam ibn Mālik’s Tashīl – Abu Hayyan’s is a famous one and his students rebuttal is not bad either.

– Imam al-Zamakshari’s works (and their explanations)

– Kāfiyatu-ibn-il-Hājib and its famous explanation.

– Kitāb-al-Sibawayh if you’re feeling adventurous.

Sarf:

– Shāfiyyatu-ibn-il-Hājib and it’s explanation.

– Al-Munsif

– Al-Mumti by ibn Usfūr.

– Al-Mustaqsa by Dr AbdulLatīf alKhatīb

Balaghah:

– Abdul-Qādir al-Jurjānīs books

– Mahmūd Shākir’s books

– Muhammad Abu Mūsa’s books – I wonder if anyone has tried to teach from these instead of classical texts?

– Tafsīr Al-Zamakshari

– Tafsīr ibn Ashūr

Adab:

– Maqamāt al-Harīrī (yes again)

– Al-Jahidh’s books especially Al-Bayān wa al-Tabyīn

– Al-Kāmil by Imam al Mubarrid.

– Al-Khasa’is of Ibn al-Jinnī

– As for poetry then refer to following link: https://youtu.be/d-FxfkAABVg

This list seems dauntingly long, but the question you need to ask yourself as an Arabic learner is: Why do I want to learn Arabic?

If you want to become a scholar, the whole list applies. If you don’t then the length of the list varies depends on what you want to achieve.

For those who want only a basic grasp of the Qur’an in Arabic so they can read small tafsirs on their own, or to engage in Modern Arab culture, then the first stage will suffice.