I’ve updated the list of multilingual sites for learning several languages together, but if you’re interested in books (some as PDFs) rather than websites, these are the resources I have:
A Comparative Practical Grammar of French, Spanish and Italian by O. W. Heatwole (1949) You may be able to buy it from third-party sellers on Amazon, but the prices tend to be very high.
This book was edited by Mario Pei and in the foreword, he explains why a book of this kind is needed:
“But how wonderful would it be if there were only a comparative grammar of the main Romance languages, that would enable me to compare at a glance a rule in the language with which I am most familiar with the corresponding rule in the language I know least!”
This work is an answer to the conscious and unconscious needs of these students and teacher of Romance languages. It is a book the necessity of which has long been felt, but somehow no one has ever taken the trouble to supply it.
There is some inconsistency in the fact that Departments of Romance Languages are far more common in our system of higher education than separate Departments of French, Spanish and Italian, yet nowhere are the Romance Languages taught as a unit from the comparative angle that would permit the learner to avoid major confusions and major pitfalls. Learning three related languages at once should certainly prove no more difficult than learning them separately.
Comparative Grammar of French, Italian, Spanish & Portuguese Languages by Edwin A. Notley (1868) is a similar book though it is much older. The obvious advantage over Heatwole’s book is the inclusion of Portuguese but since it is so old, there are a few spelling differences (Spanish mujer is muger) and probably other features that have changed in the past 144 years! The good news about this book is that it is in the public domain, and I have scanned my copy so you can download it as a PDF (or flip through it online).
The Traveller’s Manual by Karl Baedeker (1840) is another book from the 19th century that includes vocabulary and traveller’s phrases for English, German, French and Italian. It also includes some Dutch vocabulary. You can read it online through Google Books.
The Loom of Language: An Approach to the Mastery of Many Languages by Frederick Bodmer (1944) is where my love of comparative linguistics began. I reviewed the book on the blog a while ago, and it is still my favorite book overall. You can buy it through Amazon for $15 or access it online via archive.org
EuRom5 (2011) is the most recent multilingual book I’ve seen yet. It focuses on learning to read and comprehend five Romance languages. The book is written in French, Italian, Spanish, Catalan and Portuguese (so it is designed for native/advanced users of any of those languages) with texts and audio files available on the website. Unfortunately it is not available in electronic format, but you can buy it from dicoland.com or hoepli.it for under 30€. Amazon.fr also sells it for about 40€ and a few used copies are available on amazon.com but for nearly $90. I managed to buy this book in mid 2014 but have yet to really read through it. It does not appear to be as “comparative” as the other books in the list since it offers 20 articles in one language with some words glossed in the other 4 languages (i.e. the entire articles are not translated in the other languages). You can read my summary/review of it here.
Comprendre les langues romanes: Du français à l’espagnol, au portugais, à l’italien & au roumain. Méthode d’intercompréhension by Paul Teyssier (2004) is obviously written in French. It’s not available via amazon.fr; however, I ordered my copy from the Librairie Portugaise & Brésilienne in Paris for 29€, and they do ship worldwide. I believe translations of this book in the other languages exist, but I’m not sure where to buy them.
EuroComRom – The Seven Sieves: How to read all the Romance languages right away by McCann, Klein & Stegmann (2003) is a European initiative to encourage EU citizens to learn each other’s languages. It includes Spanish, Catalan, Portuguese, Italian, Romanian and French and can be bought online as a PDF for only 6€ or as a regular book for 24,50€. There is also a German version of this book which can be downloaded for free.
EuroCom has also produced a comparative Germanic book entitled Die siebe Sieben – Germanische Sprachen lesen lernen by Hufeisen and Marx (2007) that includes Dutch, Frisian, Swedish, Danish, Norwegian, and Icelandic but it is only written in German. It can also be bought online for 7,45€ or as a regular book for 29,80€.
Exploring French, German, and Spanish by Jacob Steigerwald (1987) is a neat PDF explaining the similarities of the three most commonly taught languages in the US. Download the full text for free from eric.ed.gov.
Lastly, I’ve found one book for Slavic languages, Slavische Interkomprehension: Eine Einführung, which you can buy at amazon.de. Obviously it is written in German, and it includes Bosnian/Croatian/Serbian/Montenegrin, Polish, Czech, Russian, Ukrainian. I haven’t purchased it yet, but I plan to.
Anyone know of other comparative books that teach more than one language at a time?
MOOCs for Learning Languages
Free Peace Corps Language Learning Materials: Over 100 Languages Available
How to Learn Languages by Reading Interlinear Books
Free Children's Books Apps in Foreign Languages
Comparative Grammar of the French, Italian, Spanish and Portuguese Languages
Review of Language Learning Websites II: Mango Languages, LangMaster, LinguaTV, and Yabla