gg5
smk: いつも持っていなければならない（た方が便利な）こと。また、そういう物。〔ある専門の入門書の名前に付けることが多い。例、「化学―」〕「―の書」
prog: 英会話必携  a handbook of English conversation

Book titles are sense 2. Sense 1 is usage like "学生必携の書. a book indispensable to students" or "防寒具必携のこと. Warm clothing is a must".

@@ -18,2 +18,3 @@
-<pos>&n;</pos>
-<gloss>essential (esp. handbook, guidebook, manual, etc.)</gloss>
+<pos>&n-suf;</pos>
+<gloss>indispensable</gloss>
+<gloss>essential</gloss>
@@ -24 +25 @@
-<misc>&rare;</misc>
+<s_inf>usu. in book titles</s_inf>

必携	772451	 
必携の	260221	  

#1 undocumented-in-ngram use is probably book *titles* (which is also adj-f/adj-no). After that:

From the top collocations, books:
必携情報集	77764	  
必携の書	39154	  
必携の一冊	33045	  
必携書	19266	  
必携の書籍	15285	  
必携の本	8768	  
必携本	8436	  
必携の一冊です	7468	 
必携ハンドブック	3385	  
必携マニュアル	2839	  

Possibly not books:
必携アイテム	14026	  
必携のアイテム	11174	  
必携品	7569	  
必携の機能	3300	  

Reverso is all adjective usage(I include 何々は必携だ as an adj. usage: "is indispensable" vs "is an indispensable item" ). 
https://context.reverso.net/translation/japanese-english/必携

nikk and daijs both order them this way.  sankoku doesn't give [2] at all, but its example is [科学ー・ー書]

nikk's [2] doesn't really look like a separate sense. It reads simply: ② 特に、ハンドブック。 It does not appear from ngrams that 必携 is often used by itself to mean "book"(or that it is used as any kind of noun at all):
必携の	260221
必携が	194
必携は	722
必携を	906

For that, it also seems more useful to give [1] as an adj-gloss.

Google image search shows a lot of guidebooks self-anointing themselves with this term. For something super-modern and unremarkable, see for example a guide to HTML and CSS:
Web制作必携 HTML&CSS全事典
https://book.impress.co.jp/books/1121101140

Amazon search for 必携 claims over 10,000 results... (appears almost entirely books)
https://www.amazon.co.jp/s?k=必携&crid=J8PTAZ0XSA7U&sprefix=,aps,239&ref=nb_sb_noss_2

eijiro, besides "must-have" being prevalent, adds "companion".  That's pretty interesting as a noun gloss, but can't be given as an adj. Note how this is presented though:
必携
companion〔書名に使う。「～の手引」、「必携～」など。〕

English example:
The CSS Companion
200 Concepts Every Aspirant Should Know
https://foliobooks.pk/product/the-css-companion-200-concepts-every-aspirant-should-know/

@@ -16,0 +17 @@
+<pos>&adj-no;</pos>
@@ -17,0 +19,6 @@
+<gloss>essential (esp. handbook, guidebook, manual, etc.)</gloss>
+<gloss>must-have</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>
+<misc>&rare;</misc>
@@ -21,6 +27,0 @@
-</sense>
-<sense>
-<pos>&n;</pos>
-<pos>&adj-no;</pos>
-<gloss>essential item</gloss>
-<gloss>must-have</gloss>