The title is pretty self explanatory; I am an e4 player, and I play the french defence as black to counter 1.e4.

Could anyone recommend me the best opening books for the above two openings- I want it to be thorough and have opening ideas that i will be able to play at a high level; I find my rating is very volatile at the moment so I wish to improve my chess but also play more consistantly. 

Any titles mentioned would be appreciated greatly.

"Mastering the French" by Neil McDonald and Andrew Harley.  Its out of print, but look through ebay or other sources.  Worth the price.  

This is an awesome book. It's more on formation ideas with localized tactics than theoretical variations.

You're spoilt for choice...lots of good French books have come out recently.

I hear good things about Watson's stuff. 

I think the choice depends upon what repertoire it is you want, what style of book you like (focus on variations or focus on prose), and even whether you prefer books or DVDs

The Modern French by Dejan Antic and Branimir Maksimovic is excellent

Keep it simple e4 is an amazing book

Just play D5 and start to think. On our level the opening moves aren't so important. Better start to think by your own on move 1. You will enjoy the game and get better results.

Has Journification been here since 2014? Anyway:

https://www.newinchess.com/media/wysiwyg/product_pdf/9068.pdf

https://www.chess.com/blog/HanSchut/review-keep-it-simple-1-e4-by-im-christof-sielecki

https://www.chess.com/blog/Milliern/a-solution-to-all-of-your-1-e4-problems

How to Play Against 1 e4 by Neil McDonald....

https://www.amazon.com/How-Play-Against-1-e4/dp/1857445864/ref=sr_1_11?crid=255WISC8QC0HJ&keywords=neil+mcdonald+chess&qid=1556343499&s=gateway&sprefix=neil+mcdonald%2Caps%2C204&sr=8-11

Although the book's title does not indicate it, the book is strictly about the French Defense.  Overall, IMO, an excellent introduction to the French Defense for the improving amateur player.  GM Neil McDonald is one of the better chess authors in terms of explaining plans, themes, the why's and wherefore's of what to do and how to do it.  If you are interested in learning to play the French, this is the book to start with.

Has Journification been here since 2014? Anyway:

https://web.archive.org/web/20140626174056/http://www.chesscafe.com/text/hansen120.pdf

http://marshtowers.blogspot.com/2009/01/chess-reviews-75.html

https://theweekinchess.com/john-watson-reviews/an-end-to-openings

I am enjoying "The French defense - Properly played" by Wolfgang Uhlmann. Uhlmann is a German grandmaster who has played the French throughout his career. The book consists of 76 annotated games, mostly with Uhlmann as Black; in each game Black wins. The book is well presented: not bulky, quite portable, good size print for me, not expensive, readily available from bookdepository, well edited, a few photos of famous players. My plan is to read through every game; I should be able to do this in 3 months; it takes me a day to work through a game; I also use it to answer questions about the French; e.g. to answer "How do I get to castle with the advance variation?", I go to the section on the advance variation and look through the games for some ideas on castling.

The first section deals with the Tarrasch variation 1. e4 e6 2. d4 d5 3. Nd2. Tarrasch was a giant in chess in the late 19th century-early 20th century. Now I am not a talented chess player, but I often play the French. I cannot recall any of my opponents playing this variation as White. Perhaps this is a criticism of the usefulness of this section; or perhaps it says something about opponents at my level.

I can see the point made by bananamoon that it is better to think than focus on openings. It is always better to think. In one of my current games (as Black) the opening went like this. 1. e4 e6 2. f4. Even at this point, I had to think for myself because I have not encountered 2. f4 before.

On the other hand, having a good understanding of the basics of openings such as the French defense, assists me to get through to the middle game without making a blunder.

In any case, I am enjoying the book by Uhlmann. He looks like a nice fellow in his photo.

Has Journification been here since 2014? Anyway:

https://www.newinchess.com/media/wysiwyg/product_pdf/7820.pdf